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Mathematics LibreTexts

11-1. Sequences and Their Notations

Sequences and Their Notations
In this section, you will:
  • Write the terms of a sequence defined by an explicit formula.
  • Write the terms of a sequence defined by a recursive formula.
  • Use factorial notation.

A video game company launches an exciting new advertising campaign. They predict the number of online visits to their website, or hits, will double each day. The model they are using shows 2 hits the first day, 4 hits the second day, 8 hits the third day, and so on. See [link].

Day 1 2 3 4 5
Hits 2 4 8 16 32

If their model continues, how many hits will there be at the end of the month? To answer this question, we’ll first need to know how to determine a list of numbers written in a specific order. In this section, we will explore these kinds of ordered lists.

Writing the Terms of a Sequence Defined by an Explicit Formula

One way to describe an ordered list of numbers is as a sequence. A sequence is a function whose domain is a subset of the counting numbers. The sequence established by the number of hits on the website is

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>{</mo><mn>2</mn><mo>,</mo><mn>4</mn><mo>,</mo><mn>8</mn><mo>,</mo><mn>16</mn><mo>,</mo><mn>32</mn><mo>,</mo><mo>…</mo><mo>}</mo><mo>.</mo></mrow></annotation-xml></semantics></math>

The ellipsis (…) indicates that the sequence continues indefinitely. Each number in the sequence is called a term. The first five terms of this sequence are 2, 4, 8, 16, and 32.

Listing all of the terms for a sequence can be cumbersome. For example, finding the number of hits on the website at the end of the month would require listing out as many as 31 terms. A more efficient way to determine a specific term is by writing a formula to define the sequence.

One type of formula is an explicit formula, which defines the terms of a sequence using their position in the sequence. Explicit formulas are helpful if we want to find a specific term of a sequence without finding all of the previous terms. We can use the formula to find the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence, where <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> is any positive number. In our example, each number in the sequence is double the previous number, so we can use powers of 2 to write a formula for the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term.

Sequence of {2, 4, 8, 16, 32, ...} expressed in exponential form (i.e., {2^1, 2^2, 2^3, ..., 2^n, ...}

The first term of the sequence is<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msup/></mrow></annotation-xml></semantics></math> 2 1 =2, the second term is<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msup/></mrow></annotation-xml></semantics></math> 2 2 =4, the third term is<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msup/></mrow></annotation-xml></semantics></math> 2 3 =8, and so on. The <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence can be found by raising 2 to the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> power. An explicit formula for a sequence is named by a lower case letter <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>a</mi><mo>,</mo><mi>b</mi><mo>,</mo><mi>c</mi><mn>...</mn></mrow></annotation-xml></semantics></math> with the subscript <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>.</mo></mrow></annotation-xml></semantics></math> The explicit formula for this sequence is

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2 n .

Now that we have a formula for the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence, we can answer the question posed at the beginning of this section. We were asked to find the number of hits at the end of the month, which we will take to be 31 days. To find the number of hits on the last day of the month, we need to find the 31st term of the sequence. We will substitute 31 for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> in the formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><msub><mi>a</mi></msub></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 31 = 2 31       =2,147,483,648

If the doubling trend continues, the company will get <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext>2,147,483,648</mtext></mrow></annotation-xml></semantics></math> hits on the last day of the month. That is over 2.1 billion hits! The huge number is probably a little unrealistic because it does not take consumer interest and competition into account. It does, however, give the company a starting point from which to consider business decisions.

Another way to represent the sequence is by using a table. The first five terms of the sequence and the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence are shown in [link].

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> 1 2 3 4 5 <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math>
<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n 2 4 8 16 32 <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msup><mn>2</mn></msup></mrow></annotation-xml></semantics></math> n

Graphing provides a visual representation of the sequence as a set of distinct points. We can see from the graph in [link] that the number of hits is rising at an exponential rate. This particular sequence forms an exponential function.

<figure class="small" id="CNX_Precalculus_Figure_11_01_002">Graph of a plotted exponential function, f(n) = 2^n, where the x-axis is labeled n and the y-axis is labeled a_n.</figure>

Lastly, we can write this particular sequence as

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>{</mo><mn>2</mn><mo>,</mo><mn>4</mn><mo>,</mo><mn>8</mn><mo>,</mo><mn>16</mn><mo>,</mo><mn>32</mn><mo>,</mo><mo>…</mo><mo>,</mo><msup/></mrow></annotation-xml></semantics></math> 2 n ,…}.

A sequence that continues indefinitely is called an infinite sequence. The domain of an infinite sequence is the set of counting numbers. If we consider only the first 10 terms of the sequence, we could write

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>{</mo><mn>2</mn><mo>,</mo><mn>4</mn><mo>,</mo><mn>8</mn><mo>,</mo><mn>16</mn><mo>,</mo><mn>32</mn><mo>,</mo><mo>…</mo><mo>,</mo><msup/></mrow></annotation-xml></semantics></math> 2 n ,…,1024}.

This sequence is called a finite sequence because it does not continue indefinitely.

Sequence

A sequence is a function whose domain is the set of positive integers. A finite sequence is a sequence whose domain consists of only the first <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> positive integers. The numbers in a sequence are called terms. The variable <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>a</mi></annotation-xml></semantics></math> with a number subscript is used to represent the terms in a sequence and to indicate the position of the term in the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 , a 2 , a 3 ,…, a n ,…

We call <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 the first term of the sequence, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 2 the second term of the sequence, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 3 the third term of the sequence, and so on. The term <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n is called the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence, or the general term of the sequence. An explicit formula defines the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math>term of a sequence using the position of the term. A sequence that continues indefinitely is an infinite sequence.

Does a sequence always have to begin with<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 ?

No. In certain problems, it may be useful to define the initial term as <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 0instead of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 . In these problems, the domain of the function includes 0.

Given an explicit formula, write the first<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>terms of a sequence.

  1. Substitute each value of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> into the formula. Begin with <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></annotation-xml></semantics></math> to find the first term, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 .
  2. To find the second term, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 2 , use <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>2.</mn></mrow></annotation-xml></semantics></math>
  3. Continue in the same manner until you have identified all <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> terms.
Writing the Terms of a Sequence Defined by an Explicit Formula

Write the first five terms of the sequence defined by the explicit formula <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−3n+8.

Substitute <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></annotation-xml></semantics></math> into the formula. Repeat with values 2 through 5 for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>.</mo></mrow></annotation-xml></semantics></math>

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> a 1 =−3(1)+8=5 n=2 a 2 =−3(2)+8=2 n=3 a 3 =−3(3)+8=−1 n=4 a 4 =−3(4)+8=−4 n=5 a 5 =−3(5)+8=−7

The first five terms are<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>{</mo><mn>5</mn><mo>,</mo><mtext> </mtext><mn>2</mn><mo>,</mo><mtext> </mtext><mn>−1</mn><mo>,</mo><mtext> </mtext><mn>−4</mn><mo>,</mo><mtext> </mtext><mn>−7</mn><mo>}</mo><mo>.</mo></mrow></annotation-xml></semantics></math>

Analysis

The sequence values can be listed in a table. A table, such as [link], is a convenient way to input the function into a graphing utility.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> 1 2 3 4 5
<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n 5 2 –1 –4 –7

A graph can be made from this table of values. From the graph in [link], we can see that this sequence represents a linear function, but notice the graph is not continuous because the domain is over the positive integers only.

<figure id="CNX_Precalculus_Figure_11_01_003">Graph of a scattered plot where the x-axis is labeled n and the y-axis is labeled a_n.</figure>

Write the first five terms of the sequence defined by the explicit formula<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> t n =5n−4.

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 1,6, 11, 16, 21 }.

Investigating Alternating Sequences

Sometimes sequences have terms that are alternate. In fact, the terms may actually alternate in sign. The steps to finding terms of the sequence are the same as if the signs did not alternate. However, the resulting terms will not show increase or decrease as <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi></mrow></annotation-xml></semantics></math> increases. Let’s take a look at the following sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>{</mo><mn>2</mn><mo>,</mo><mn>−4</mn><mo>,</mo><mn>6</mn><mo>,</mo><mn>−8</mn><mo>}</mo></mrow></annotation-xml></semantics></math>

Notice the first term is greater than the second term, the second term is less than the third term, and the third term is greater than the fourth term. This trend continues forever. Do not rearrange the terms in numerical order to interpret the sequence.

Given an explicit formula with alternating terms, write the first <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> terms of a sequence.

  1. Substitute each value of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> into the formula. Begin with <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></annotation-xml></semantics></math> to find the first term, <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 . The sign of the term is given by the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msup><mrow><mrow><mo>(</mo></mrow></mrow></msup></mrow></annotation-xml></semantics></math> −1 ) n in the explicit formula.
  2. To find the second term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 2 , use<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>2.</mn><mtext> </mtext></mrow></annotation-xml></semantics></math>
  3. Continue in the same manner until you have identified all <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi></mrow></annotation-xml></semantics></math> terms.
Writing the Terms of an Alternating Sequence Defined by an Explicit Formula

Write the first five terms of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = (−1) n n 2 n+1

Substitute <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo></mrow></annotation-xml></semantics></math> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>2</mn><mo>,</mo></mrow></annotation-xml></semantics></math> and so on in the formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> a 1 = (−1) 1 2 2 1+1 =− 1 2 n=2 a 2 = (−1) 2 2 2 2+1 = 4 3 n=3 a 3 = (−1) 3 3 2 3+1 =− 9 4 n=4 a 4 = (−1) 4 4 2 4+1 =16 5 n=5 a 5 = (−1) 5 5 2 5+1 =− 25 6

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo><mrow/></mrow></mrow></annotation-xml></semantics></math> − 1 2 , 4 3 ,− 9 4 , 16 5 ,− 25 6 }.

Analysis

The graph of this function, shown in [link], looks different from the ones we have seen previously in this section because the terms of the sequence alternate between positive and negative values.

<figure id="CNX_Precalculus_Figure_11_01_004">Graph of a scattered plot with labeled points: (1, -1/2), (2, 4/3), (3, -9/4), (4, 16/5), and (5, -25/6). The x-axis is labeled n and the y-axis is labeled a_n.</figure>

In [link], does the (–1) to the power of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> account for the oscillations of signs?

Yes, the power might be <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>,</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo>,</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>and so on, but any odd powers will result in a negative term, and any even power will result in a positive term.

Write the first five terms of the sequence:

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 4n (−2) n

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> −2, 2, − 3 2 , 1, − 5 8 }.

Investigating Piecewise Explicit Formulas

We’ve learned that sequences are functions whose domain is over the positive integers. This is true for other types of functions, including some piecewise functions. Recall that a piecewise function is a function defined by multiple subsections. A different formula might represent each individual subsection.

Given an explicit formula for a piecewise function, write the first<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>terms of a sequence

  1. Identify the formula to which <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></annotation-xml></semantics></math> applies.
  2. To find the first term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 , use<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>1</mn><mtext> </mtext></mrow></annotation-xml></semantics></math>in the appropriate formula.
  3. Identify the formula to which<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>2</mn><mtext> </mtext></mrow></annotation-xml></semantics></math>applies.
  4. To find the second term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 2 , use<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>2</mn><mtext> </mtext></mrow></annotation-xml></semantics></math>in the appropriate formula.
  5. Continue in the same manner until you have identified all<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>terms.
Writing the Terms of a Sequence Defined by a Piecewise Explicit Formula

Write the first six terms of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ n 2 if n is not divisible by 3 n 3 if n is divisible by 3

Substitute<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo><mi>n</mi><mo>=</mo><mn>2</mn><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>and so on in the appropriate formula. Use <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msup><mi>n</mi></msup></mrow></annotation-xml></semantics></math> 2 when <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> is not a multiple of 3. Use <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mi>n</mi></mfrac></mrow></annotation-xml></semantics></math> 3 when <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> is a multiple of 3.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><msub><mi>a</mi></msub></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 1 = 1 2 =1 1 is not a multiple of 3.  Use  n 2 . a 2 = 2 2 =4 2 is not a multiple of 3.  Use  n 2 . a 3 = 3 3 =13 is a multiple of 3.  Use  n 3 . a 4 = 4 2 =16 4 is not a multiple of 3.  Use  n 2 . a 5 = 5 2 =25 5 is not a multiple of 3.  Use  n2 . a 6 = 6 3 =2 6 is a multiple of 3.  Use  n 3 .

The first six terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 1, 4, 1, 16, 25, 2 }.

Analysis

Every third point on the graph shown in [link] stands out from the two nearby points. This occurs because the sequence was defined by a piecewise function.

<figure id="CNX_Precalculus_Figure_11_01_005">Graph of a scattered plot where the x-axis is labeled n and the y-axis is labeled a_n.</figure>

Write the first six terms of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ 2 n 3 if n is odd 5n 2 if n is even

The first six terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 2, 5, 54, 10, 250, 15 }.

Finding an Explicit Formula

Thus far, we have been given the explicit formula and asked to find a number of terms of the sequence. Sometimes, the explicit formula for the<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext>th</mtext><mtext> </mtext></mrow></annotation-xml></semantics></math>term of a sequence is not given. Instead, we are given several terms from the sequence. When this happens, we can work in reverse to find an explicit formula from the first few terms of a sequence. The key to finding an explicit formula is to look for a pattern in the terms. Keep in mind that the pattern may involve alternating terms, formulas for numerators, formulas for denominators, exponents, or bases.

Given the first few terms of a sequence, find an explicit formula for the sequence.

  1. Look for a pattern among the terms.
  2. If the terms are fractions, look for a separate pattern among the numerators and denominators.
  3. Look for a pattern among the signs of the terms.
  4. Write a formula for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n in terms of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>.</mo></mrow></annotation-xml></semantics></math> Test your formula for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mi>n</mi><mo>=</mo><mn>2</mn><mo>,</mo></mrow></annotation-xml></semantics></math> and <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>3.</mn></mrow></annotation-xml></semantics></math>
Writing an Explicit Formula for the nth Term of a Sequence

Write an explicit formula for the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of each sequence.

  1. <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> − 2 11 , 3 13 ,− 4 15 , 5 17 ,− 6 19 ,… }
  2. <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>{</mo><mo>−</mo><mfrac/></mrow></annotation-xml></semantics></math> 2 25 ,− 2 125 ,− 2 625 ,− 2 3,125 ,− 2 15,625 ,…}
  3. <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>{</mo><msup/></mrow></annotation-xml></semantics></math> e 4 , e 5 , e 6 , e 7 , e 8 ,…}

Look for the pattern in each sequence.

  1. The terms alternate between positive and negative. We can use<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msup/></mrow></annotation-xml></semantics></math> (−1) n  to make the terms alternate. The numerator can be represented by <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>+</mo><mn>1.</mn></mrow></annotation-xml></semantics></math> The denominator can be represented by <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>2</mn><mi>n</mi><mo>+</mo><mn>9.</mn></mrow></annotation-xml></semantics></math>

    <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = (−1) n (n+1) 2n+9

  2. The terms are all negative.

    So we know that the fraction is negative, the numerator is 2, and the denominator can be represented by <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msup><mn>5</mn></msup></mrow></annotation-xml></semantics></math> n+1 .

    <math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =− 2 5 n+1
  3. The terms are powers of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>e</mi><mo>.</mo></mrow></annotation-xml></semantics></math> For <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo></mrow></annotation-xml></semantics></math> the first term is <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msup><mi>e</mi></msup></mrow></annotation-xml></semantics></math> 4 so the exponent must be <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>+</mo><mn>3.</mn></mrow></annotation-xml></semantics></math>

    <math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = e n+3

Write an explicit formula for the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext>{9,</mtext><mtext> </mtext><mo>−</mo><mtext>81,</mtext><mtext> </mtext><mtext>729,</mtext><mtext> </mtext><mo>−</mo><mtext>6,561,</mtext><mtext> </mtext><mtext>59,049,</mtext><mtext> </mtext><mtext>…}</mtext></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = (−1) n+1 9 n

Write an explicit formula for the<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext>th</mtext><mtext> </mtext></mrow></annotation-xml></semantics></math>term of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> − 3 4 ,− 9 8 ,− 27 12 ,− 81 16 ,− 243 20 ,... }

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =− 3 n 4n

Write an explicit formula for the<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext>th</mtext><mtext> </mtext></mrow></annotation-xml></semantics></math>term of the sequence.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 1 e 2 ,  1 e , 1, e,  e 2 ,... }

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = e n−3

Writing the Terms of a Sequence Defined by a Recursive Formula

Sequences occur naturally in the growth patterns of nautilus shells, pinecones, tree branches, and many other natural structures. We may see the sequence in the leaf or branch arrangement, the number of petals of a flower, or the pattern of the chambers in a nautilus shell. Their growth follows the Fibonacci sequence, a famous sequence in which each term can be found by adding the preceding two terms. The numbers in the sequence are 1, 1, 2, 3, 5, 8, 13, 21, 34,…. Other examples from the natural world that exhibit the Fibonacci sequence are the Calla Lily, which has just one petal, the Black-Eyed Susan with 13 petals, and different varieties of daisies that may have 21 or 34 petals.

Each term of the Fibonacci sequence depends on the terms that come before it. The Fibonacci sequence cannot easily be written using an explicit formula. Instead, we describe the sequence using a recursive formula, a formula that defines the terms of a sequence using previous terms.

A recursive formula always has two parts: the value of an initial term (or terms), and an equation defining <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n in terms of preceding terms. For example, suppose we know the following:

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><msub><mi>a</mi></msub></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 1 =3 a n =2 a n−1 −1, for n≥2

We can find the subsequent terms of the sequence using the first term.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mtable columnalign="left"><mtr><mtd><msub><mi>a</mi></msub></mtd></mtr></mtable></annotation-xml></semantics></math> 1 =3 a 2 =2 a 1 −1=2(3)−1=5 a 3 =2 a 2 −1=2(5)−1=9 a 4 =2 a 3 −1=2(9)−1=17

So the first four terms of the sequence are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 3, 5, 9, 17 }.

The recursive formula for the Fibonacci sequence states the first two terms and defines each successive term as the sum of the preceding two terms.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><msub><mi>a</mi></msub></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 1 =1 a 2 =1 a n = a n−1 + a n−2 , for  n≥3

To find the tenth term of the sequence, for example, we would need to add the eighth and ninth terms. We were told previously that the eighth and ninth terms are 21 and 34, so

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 10 = a 9 + a 8 =34+21=55
Recursive Formula

A recursive formula is a formula that defines each term of a sequence using preceding term(s). Recursive formulas must always state the initial term, or terms, of the sequence.

Must the first two terms always be given in a recursive formula?

No. The Fibonacci sequence defines each term using the two preceding terms, but many recursive formulas define each term using only one preceding term. These sequences need only the first term to be defined.

Given a recursive formula with only the first term provided, write the first <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> terms of a sequence.

  1. Identify the initial term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 , which is given as part of the formula. This is the first term.
  2. To find the second term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 2 , substitute the initial term into the formula for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n−1 . Solve.
  3. To find the third term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 3 , substitute the second term into the formula. Solve.
  4. Repeat until you have solved for the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term.
Writing the Terms of a Sequence Defined by a Recursive Formula

Write the first five terms of the sequence defined by the recursive formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mtable columnalign="left"><mtr><mtd><mrow/></mtd></mtr><mtr><mtd><msub><mi>a</mi></msub></mtd></mtr></mtable></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 1 =9 a n =3 a n−1 −20, for n≥2

The first term is given in the formula. For each subsequent term, we replace <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n−1 with the value of the preceding term.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mtable columnalign="left"/></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> a 1 =9 n=2 a 2 =3 a 1 −20=3(9)−20=27−20=7 n=3 a 3 =3 a 2 −20=3(7)−20=21−20=1 n=4 a 4 =3 a 3−20=3(1)−20=3−20=−17 n=5 a 5 =3 a 4 −20=3(−17)−20=−51−20=−71

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 9, 7, 1, –17, –71 }. See [link].

<figure id="Figure_11_01_006">Graph of a scattered plot with labeled points: (1, 9), (2, 7), (3, 1), (4, -17), and (5, -71). The x-axis is labeled n and the y-axis is labeled a_n.</figure>

Write the first five terms of the sequence defined by the recursive formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mtable columnalign="left"><mtr><mtd><msub><mi>a</mi></msub></mtd></mtr></mtable></annotation-xml></semantics></math> 1 =2 a n =2 a n−1 +1, for n≥2

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 2, 5, 11, 23, 47 }

Given a recursive formula with two initial terms, write the first <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> terms of a sequence.

  1. Identify the initial term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 , which is given as part of the formula.
  2. Identify the second term,<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 2 , which is given as part of the formula.
  3. To find the third term, substitute the initial term and the second term into the formula. Evaluate.
  4. Repeat until you have evaluated the <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mtext>th</mtext></mrow></annotation-xml></semantics></math> term.
Writing the Terms of a Sequence Defined by a Recursive Formula

Write the first six terms of the sequence defined by the recursive formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mtable columnalign="left"><mtr><mtd><msub><mi>a</mi></msub></mtd></mtr></mtable></annotation-xml></semantics></math> 1 =1 a 2 =2 a n =3 a n−1 +4 a n−2 , for n≥3

The first two terms are given. For each subsequent term, we replace <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n−1 and <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n−2 with the values of the two preceding terms.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mi>n</mi><mo>=</mo><mn>3</mn></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> a 3 =3 a 2 +4 a 1 =3(2)+4(1)=10 n=4 a 4 =3 a 3 +4 a 2 =3(10)+4(2)=38 n=5 a 5 =3 a 4 +4 a 3 =3(38)+4(10)=154 n=6 a 6=3 a 5 +4 a 4 =3(154)+4(38)=614

The first six terms are<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mtext>{1,2,10,38,154,614}</mtext><mtext>.</mtext></mrow></annotation-xml></semantics></math> See [link].

<figure id="CNX_Precalculus_Figure_11_01_007">Graph of a scattered plot with labeled points: (1, 1), (2, 2), (3, 10), (4, 38), (5, 154) and (6, 614). The x-axis is labeled n and the y-axis is labeled a_n.</figure>

Write the first 8 terms of the sequence defined by the recursive formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mtable columnalign="left"><mtr><mtd><mrow/></mtd></mtr><mtr><mtd><msub><mi>a</mi></msub></mtd></mtr></mtable></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 1 =0 a 2 =1 a 3 =1 a n = a n−1 a n−2 + a n−3 , for n≥4

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 0, 1, 1, 1, 2, 3,  5 2 ,  17 6 }.

Using Factorial Notation

The formulas for some sequences include products of consecutive positive integers. <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> factorial, written as <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>!</mo><mo>,</mo></mrow></annotation-xml></semantics></math> is the product of the positive integers from 1 to <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>.</mo></mrow></annotation-xml></semantics></math> For example,

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mn>4</mn><mo>!</mo><mo>=</mo><mn>4</mn><mo>⋅</mo><mn>3</mn><mo>⋅</mo><mn>2</mn><mo>⋅</mo><mn>1</mn><mo>=</mo><mn>24</mn></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> 5!=5⋅4⋅3⋅2⋅1=120

An example of formula containing a factorial is <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =(n+1)!. The sixth term of the sequence can be found by substituting 6 for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>.</mo></mrow></annotation-xml></semantics></math>

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 6 =(6+1)!=7!=7·6·5·4·3·2·1=5040

The factorial of any whole number <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> is <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo stretchy="false">(</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo stretchy="false">)</mo><mo>!</mo></mrow></annotation-xml></semantics></math> We can therefore also think of <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>5</mn><mo>!</mo></mrow></annotation-xml></semantics></math> as<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mn>5</mn><mo>⋅</mo><mn>4</mn><mo>!</mo><mtext>.</mtext></mrow></annotation-xml></semantics></math>

Factorial

n factorial is a mathematical operation that can be defined using a recursive formula. The factorial of<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>denoted<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>!</mo><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>is defined for a positive integer <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi></mrow></annotation-xml></semantics></math> as:

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mtable columnalign="left"><mtr><mtd><mn>0</mn><mo>!</mo><mo>=</mo><mn>1</mn></mtd></mtr></mtable></annotation-xml></semantics></math> 1!=1 n!=n( n−1 )( n−2 )⋯( 2 )( 1 ), for n≥2

The special case <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>0</mn><mo>!</mo></mrow></annotation-xml></semantics></math> is defined as <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>0</mn><mo>!</mo><mo>=</mo><mn>1.</mn></mrow></annotation-xml></semantics></math>

Can factorials always be found using a calculator?

No. Factorials get large very quickly—faster than even exponential functions! When the output gets too large for the calculator, it will not be able to calculate the factorial.

Writing the Terms of a Sequence Using Factorials

Write the first five terms of the sequence defined by the explicit formula <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 5n (n+2)! .

Substitute<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo><mi>n</mi><mo>=</mo><mn>2</mn><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>and so on in the formula.

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></mtd></mtr></mtable></mrow></annotation-xml></semantics></math> a 1 = 5(1) (1+2)! = 5 3! = 5 3·2·1 = 5 6 n=2 a 2 = 5(2) (2+2)! = 10 4! = 10 4·3·2·1 = 5 12 n=3 a 3 = 5(3) (3+2)! = 15 5!= 15 5·4·3·2·1 = 1 8 n=4 a 4 = 5(4) (4+2)! = 20 6! = 20 6·5·4·3·2·1 = 1 36 n=5 a 5 = 5(5) (5+2)! = 25 7! = 25 7·6·5·4·3·2·1= 5 1,008

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>{</mo><mfrac/></mrow></annotation-xml></semantics></math> 5 6 , 5 12 , 1 8 , 1 36 , 5 1,008 }.

Analysis

[link] shows the graph of the sequence. Notice that, since factorials grow very quickly, the presence of the factorial term in the denominator results in the denominator becoming much larger than the numerator as <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi></mrow></annotation-xml></semantics></math> increases. This means the quotient gets smaller and, as the plot of the terms shows, the terms are decreasing and nearing zero.

<figure id="CNX_Precalculus_Figure_11_01_008">Graph of a scattered plot with labeled points: (1, 5/6), (2, 5/12), (3, 1/8), (4, 1/36),  and (5, 5/1008). The x-axis is labeled n and the y-axis is labeled a_n.</figure>

Write the first five terms of the sequence defined by the explicit formula <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = (n+1)! 2n .

The first five terms are <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>{</mo></mrow></mrow></annotation-xml></semantics></math> 1,  3 2 , 4, 15, 72 }.

Access this online resource for additional instruction and practice with sequences.

Key Equations

Formula for a factorial <math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mtable columnalign="left"><mtr><mtd><mn>0</mn><mo>!</mo><mo>=</mo><mn>1</mn></mtd></mtr></mtable></annotation-xml></semantics></math> 1!=1 n!=n( n−1 )( n−2 )⋯( 2 )( 1 ), for n≥2

Key Concepts

  • A sequence is a list of numbers, called terms, written in a specific order.
  • Explicit formulas define each term of a sequence using the position of the term. See [link], [link], and [link].
  • An explicit formula for the<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext>th</mtext><mtext> </mtext></mrow></annotation-xml></semantics></math>term of a sequence can be written by analyzing the pattern of several terms. See [link].
  • Recursive formulas define each term of a sequence using previous terms.
  • Recursive formulas must state the initial term, or terms, of a sequence.
  • A set of terms can be written by using a recursive formula. See [link] and [link].
  • A factorial is a mathematical operation that can be defined recursively.
  • The factorial of<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>is the product of all integers from 1 to<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>See [link].

Section Exercises

Verbal

Discuss the meaning of a sequence. If a finite sequence is defined by a formula, what is its domain? What about an infinite sequence?

A sequence is an ordered list of numbers that can be either finite or infinite in number. When a finite sequence is defined by a formula, its domain is a subset of the non-negative integers. When an infinite sequence is defined by a formula, its domain is all positive or all non-negative integers.

Describe three ways that a sequence can be defined.

Is the ordered set of even numbers an infinite sequence? What about the ordered set of odd numbers? Explain why or why not.

Yes, both sets go on indefinitely, so they are both infinite sequences.

What happens to the terms <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n of a sequence when there is a negative factor in the formula that is raised to a power that includes <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>?</mo></mrow></annotation-xml></semantics></math> What is the term used to describe this phenomenon?

What is a factorial, and how is it denoted? Use an example to illustrate how factorial notation can be beneficial.

A factorial is the product of a positive integer and all the positive integers below it. An exclamation point is used to indicate the operation. Answers may vary. An example of the benefit of using factorial notation is when indicating the product It is much easier to write than it is to write out <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext>13</mtext><mo>⋅</mo><mtext>12</mtext><mo>⋅</mo><mtext>11</mtext><mo>⋅</mo><mtext>10</mtext><mo>⋅</mo><mtext>9</mtext><mo>⋅</mo><mtext>8</mtext><mo>⋅</mo><mtext>7</mtext><mo>⋅</mo><mtext>6</mtext><mo>⋅</mo><mtext>5</mtext><mo>⋅</mo><mtext>4</mtext><mo>⋅</mo><mtext>3</mtext><mo>⋅</mo><mtext>2</mtext><mo>⋅</mo><mtext>1</mtext><mtext>.</mtext></mrow></annotation-xml></semantics></math>

Algebraic

For the following exercises, write the first four terms of the sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2 n −2

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =− 16 n+1

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>8</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mfrac/></mrow></annotation-xml></semantics></math> 16 3 , −4, − 16 5

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =− ( −5 ) n−1

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2 n n 3

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>2</mn><mo>,</mo><mtext> </mtext><mfrac/></mrow></annotation-xml></semantics></math> 1 2 ,  8 27 ,  1 4.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2n+1 n 3

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =1.25⋅ ( −4 ) n−1

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>1.25</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>5</mn><mo>,</mo><mtext> </mtext><mn>20</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>80</mn></mrow></annotation-xml></semantics></math>.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−4⋅ ( −6 ) n−1

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n 2 2n+1

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mn>1</mn></mfrac></mrow></annotation-xml></semantics></math> 3 ,  4 5 ,  9 7 ,  16 9.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = ( −10 ) n +1

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−( 4⋅ (−5) n−1 5 )

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mfrac/></mrow></annotation-xml></semantics></math> 4 5 , 4, −20, 100

For the following exercises, write the first eight terms of the piecewise sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ (−2) n −2 if n is even (3) n−1 if n is odd

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ n 2 2n+1 if n ≤5 n 2 −5 if n >5

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mn>1</mn></mfrac></mrow></annotation-xml></semantics></math> 3 ,  4 5 ,  9 7 ,  16 9 ,  25 11 , 31, 44, 59

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ (2n+1) 2 if n is divisible by 4 2 n if n is not divisible by 4

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ −0.6⋅ 5 n−1 if n is prime or 1 2.5⋅ (−2) n−1 if n is composite

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>0.6</mn><mo>,</mo><mo>−</mo><mn>3</mn><mo>,</mo><mo>−</mo><mn>15</mn><mo>,</mo><mo>−</mo><mn>20</mn><mo>,</mo><mo>−</mo><mn>375</mn><mo>,</mo><mo>−</mo><mn>80</mn><mo>,</mo><mo>−</mo><mn>9375</mn><mo>,</mo><mo>−</mo><mn>320</mn></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ 4( n 2 −2) if n ≤3 or n > 6 n 2 −2 4 if 3<n≤6

For the following exercises, write an explicit formula for each sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>4</mn><mo>,</mo><mo> </mo><mn>7</mn><mo>,</mo><mo> </mo><mn>12</mn><mo>,</mo><mo> </mo><mn>19</mn><mo>,</mo><mo> </mo><mn>28</mn><mo>,</mo><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n 2 +3

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>4</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>−</mo><mn>10</mn><mo>,</mo><mn>14</mn><mo>,</mo><mo>−</mo><mn>34</mn><mo>,</mo><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>1</mn><mo>,</mo><mn>1</mn><mo>,</mo><mfrac/></mrow></annotation-xml></semantics></math> 4 3 ,2, 16 5 ,…

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2 n 2n  or  2 n−1 n

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>0</mn><mo>,</mo><mfrac/></mrow></annotation-xml></semantics></math> 1− e 1 1+ e 2 , 1− e 2 1+ e 3 , 1− e 3 1+ e 4 , 1− e 4 1+ e 5 ,…

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>1</mn><mo>,</mo><mo>−</mo><mfrac/></mrow></annotation-xml></semantics></math> 1 2 , 1 4 ,− 1 8 , 1 16 ,…

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = ( − 1 2 ) n−1

For the following exercises, write the first five terms of the sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =9,  a n = a n−1 +n

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =3,  a n =( −3 ) a n−1

First five terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>3</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>9</mn><mo>,</mo><mtext> </mtext><mn>27</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>81</mn><mo>,</mo><mtext> </mtext><mn>243</mn></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =−4,  a n = a n−1 +2n a n−1 −1

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =−1,  a n = ( −3 ) n−1 a n−1 −2

First five terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mn>1</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>9</mn><mo>,</mo><mtext> </mtext><mfrac/></mrow></annotation-xml></semantics></math> 27 11 ,  891 5

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =−30,  a n =( 2+ a n−1 ) ( 1 2 ) n

For the following exercises, write the first eight terms of the sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 = 1 24 ,  a 2 =1,  a n =( 2 a n−2 )( 3 a n−1 )

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mn>1</mn></mfrac></mrow></annotation-xml></semantics></math> 24 , 1,  1 4 ,  3 2 ,  9 4 ,  81 4 ,  2187 8 ,  531,441 16

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =−1,  a 2 =5,  a n = a n−2 ( 3− a n−1 )

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =2,  a 2 =10,  a n = 2( a n−1 +2 ) a n−2

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>2</mn><mo>,</mo><mtext> </mtext><mn>10</mn><mo>,</mo><mtext> </mtext><mn>12</mn><mo>,</mo><mtext> </mtext><mfrac/></mrow></annotation-xml></semantics></math> 14 5 ,  4 5 , 2, 10, 12

For the following exercises, write a recursive formula for each sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>2.5</mn><mo>,</mo><mo>−</mo><mn>5</mn><mo>,</mo><mo>−</mo><mn>10</mn><mo>,</mo><mo>−</mo><mn>20</mn><mo>,</mo><mo>−</mo><mn>40</mn><mo>,</mo><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>8</mn><mo>,</mo><mo>−</mo><mn>6</mn><mo>,</mo><mo>−</mo><mn>3</mn><mo>,</mo><mn>1</mn><mo>,</mo><mn>6</mn><mo>,</mo><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =−8, a n = a n−1 +n

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>2</mn><mo>,</mo><mtext> </mtext><mn>4</mn><mo>,</mo><mtext> </mtext><mn>12</mn><mo>,</mo><mtext> </mtext><mn>48</mn><mo>,</mo><mtext> </mtext><mn>240</mn><mo>,</mo><mtext> </mtext><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>35</mn><mo>,</mo><mtext> </mtext><mn>38</mn><mo>,</mo><mtext> </mtext><mn>41</mn><mo>,</mo><mtext> </mtext><mn>44</mn><mo>,</mo><mtext> </mtext><mn>47</mn><mo>,</mo><mtext> </mtext><mo>…</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =35, a n = a n−1 +3

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>15</mn><mo>,</mo><mn>3</mn><mo>,</mo><mfrac/></mrow></annotation-xml></semantics></math> 3 5 , 3 25 , 3 125 ,⋯

For the following exercises, evaluate the factorial.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>6</mn><mo>!</mo></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>720</mn></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mrow><mo>(</mo></mrow></mrow></annotation-xml></semantics></math> 12 6 )!

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mrow><mn>12</mn><mo>!</mo></mrow></mfrac></mrow></annotation-xml></semantics></math> 6!

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>665</mn><mo>,</mo><mn>280</mn></mrow></annotation-xml></semantics></math>

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mrow><mn>100</mn><mo>!</mo></mrow></mfrac></mrow></annotation-xml></semantics></math> 99!

For the following exercises, write the first four terms of the sequence.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n! n 2

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>1</mn><mo>,</mo><mfrac/></mrow></annotation-xml></semantics></math> 1 2 , 2 3 , 3 2

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 3⋅n! 4⋅n!

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n! n 2 −n−1

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mfrac/></mrow></annotation-xml></semantics></math> 6 5 , 24 11

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 100⋅n n(n−1)!

Graphical

For the following exercises, graph the first five terms of the indicated sequence

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = ( −1 ) n n +n

Graph of a scattered plot with points at (1, 0), (2, 5/2), (3, 8/3), (4, 17/4), and (5, 24/5). The x-axis is labeled n and the y-axis is labeled a_n.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n ={ 4+n 2n if n is even 3+n if n is odd

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =2,  a n = ( − a n−1 +1 ) 2

Graph of a scattered plot with points at (1, 2), (2, 1), (3, 0), (4, 1), and (5, 0). The x-axis is labeled n and the y-axis is labeled a_n.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =1,  a n = a n−1 +8

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = ( n+1 )! ( n−1 )!

Graph of a scattered plot with labeled points: (1, 2), (2, 6), (3, 12), (4, 20), and (5, 30). The x-axis is labeled n and the y-axis is labeled a_n.

For the following exercises, write an explicit formula for the sequence using the first five points shown on the graph.

Graph of a scattered plot with labeled points: (1, 5), (2, 7), (3, 9), (4, 11), and (5, 13). The x-axis is labeled n and the y-axis is labeled a_n.
Graph of a scattered plot with labeled points: (1, 0.5), (2, 1), (3, 2), (4, 4), and (5, 8). The x-axis is labeled n and the y-axis is labeled a_n.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 2 n−2

Graph of a scattered plot with labeled points: (1, 12), (2, 9), (3, 6), (4, 3), and (5, 0). The x-axis is labeled n and the y-axis is labeled a_n.

For the following exercises, write a recursive formula for the sequence using the first five points shown on the graph.

Graph of a scattered plot with labeled points: (1, 6), (2, 7), (3, 9), (4, 13), and (5, 21). The x-axis is labeled n and the y-axis is labeled a_n.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =6,  a n =2 a n−1 −5

Graph of a scattered plot with labeled points: (1, 16), (2, 8), (3, 4), (4, 2), and (5, 1). The x-axis is labeled n and the y-axis is labeled a_n.

Technology

Follow these steps to evaluate a sequence defined recursively using a graphing calculator:

  • On the home screen, key in the value for the initial term<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1  and press [ENTER].
  • Enter the recursive formula by keying in all numerical values given in the formula, along with the key strokes [2ND] ANS for the previous term<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a n−1 . Press [ENTER].
  • Continue pressing [ENTER] to calculate the values for each successive term.

For the following exercises, use the steps above to find the indicated term or terms for the sequence.

Find the first five terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 = 87 111 ,  a n = 4 3 a n−1 + 12 37 . Use the >Frac feature to give fractional results.

First five terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mrow><mn>29</mn></mrow></mfrac></mrow></annotation-xml></semantics></math> 37 , 152 111 , 716 333 , 3188 999 , 13724 2997

Find the 15th term of the sequence<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 =625,  a n =0.8 a n−1 +18.

Find the first five terms of the sequence<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a 1 =2,  a n = 2 [( a n −1)−1] +1.

First five terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>5</mn><mo>,</mo><mn>17</mn><mo>,</mo><mn>65537</mn></mrow></annotation-xml></semantics></math>

Find the first ten terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =8,  a n = ( a n−1 +1 )! a n−1 ! .

Find the tenth term of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =2,  a n =n a n−1

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 10 =7,257,600

Follow these steps to evaluate a finite sequence defined by an explicit formula. Using a TI-84, do the following.

  • In the home screen, press [2ND] LIST.
  • Scroll over to OPS and choose “seq(” from the dropdown list. Press [ENTER].
  • In the line headed “Expr:” type in the explicit formula, using the<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo stretchy="false">[</mo><mtext>X,T</mtext><mo>,</mo><mi>θ</mi><mo>,</mo><mi>n</mi><mo stretchy="false">]</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>button for<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi></mrow></annotation-xml></semantics></math>
  • In the line headed “Variable:” type in the variable used on the previous step.
  • In the line headed “start:” key in the value of<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>that begins the sequence.
  • In the line headed “end:” key in the value of<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi><mtext> </mtext></mrow></annotation-xml></semantics></math>that ends the sequence.
  • Press [ENTER] 3 times to return to the home screen. You will see the sequence syntax on the screen. Press [ENTER]to see the list of terms for the finite sequence defined. Use the right arrow key to scroll through the list of terms.

Using a TI-83, do the following.

  • In the home screen, press [2ND] LIST.
  • Scroll over to OPS and choose “seq(” from the dropdown list. Press [ENTER].
  • Enter the items in the order “Expr”, “Variable”, “start”, “end” separated by commas. See the instructions above for the description of each item.
  • Press [ENTER] to see the list of terms for the finite sequence defined. Use the right arrow key to scroll through the list of terms.

For the following exercises, use the steps above to find the indicated terms for the sequence. Round to the nearest thousandth when necessary.

List the first five terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =− 28 9 n+ 5 3 .

List the first six terms of the sequence<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a n = n 3 −3.5 n 2 + 4.1n−1.5 2.4n .

First six terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>0.042</mn><mo>,</mo><mn>0.146</mn><mo>,</mo><mn>0.875</mn><mo>,</mo><mn>2.385</mn><mo>,</mo><mn>4.708</mn></mrow></annotation-xml></semantics></math>

List the first five terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 15n⋅ ( −2 ) n−1 47

List the first four terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = 5.7 n +0.275( n−1 )!

First four terms: <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>5.975</mn><mo>,</mo><mn>32.765</mn><mo>,</mo><mn>185.743</mn><mo>,</mo><mn>1057.25</mn><mo>,</mo><mn>6023.521</mn></mrow></annotation-xml></semantics></math>

List the first six terms of the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n! n .

Extensions

Consider the sequence defined by <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−6−8n. Is <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−421 a term in the sequence? Verify the result.

If<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a n =−421 is a term in the sequence, then solving the equation <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mo>−</mo><mn>421</mn><mo>=</mo><mo>−</mo><mn>6</mn><mo>−</mo><mn>8</mn><mi>n</mi></mrow></annotation-xml></semantics></math> for <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mi>n</mi></annotation-xml></semantics></math> will yield a non-negative integer. However, if<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>−</mo><mn>421</mn><mo>=</mo><mo>−</mo><mn>6</mn><mo>−</mo><mn>8</mn><mi>n</mi><mo>,</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>then <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mi>n</mi><mo>=</mo><mn>51.875</mn></mrow></annotation-xml></semantics></math> so <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n =−421 is not a term in the sequence.

What term in the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = n 2 +4n+4 2( n+2 ) has the value <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>41</mn><mo>?</mo></mrow></annotation-xml></semantics></math> Verify the result.

Find a recursive formula for the sequence <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mn>1</mn><mo>,</mo><mtext> </mtext><mn>0</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mn>0</mn><mo>,</mo><mtext> </mtext><mn>1</mn><mo>,</mo><mtext> </mtext><mn>1</mn><mo>,</mo><mtext> </mtext><mn>0</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mo>−</mo><mn>1</mn><mo>,</mo><mtext> </mtext><mn>0</mn><mo>,</mo><mtext> </mtext><mn>1</mn><mo>,</mo><mtext> </mtext><mn>1</mn><mo>,</mo><mtext> </mtext><mn>...</mn><mtext> </mtext><mo>.</mo></mrow></annotation-xml></semantics></math> (Hint: find a pattern for<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><msub/></mrow></annotation-xml></semantics></math> a n  based on the first two terms.)

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> 1 =1, a 2 =0, a n = a n−1 − a n−2

Calculate the first eight terms of the sequences <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>a</mi></msub></mrow></annotation-xml></semantics></math> n = ( n+2 )! ( n−1 )! and <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><msub><mi>b</mi></msub></mrow></annotation-xml></semantics></math> n = n 3 +3 n 2 +2n, and then make a conjecture about the relationship between these two sequences.

Prove the conjecture made in the preceding exercise.

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mfrac><mrow><mo stretchy="false">(</mo><mi>n</mi><mo>+</mo><mn>2</mn><mo stretchy="false">)</mo><mo>!</mo></mrow></mfrac></mrow></annotation-xml></semantics></math> (n−1)! = (n+2)·(n+1)·(n)·(n−1)·...·3·2·1 (n−1)·...·3·2·1 =n(n+1)(n+2)= n 3 +3 n 2 +2n

Glossary

explicit formula
a formula that defines each term of a sequence in terms of its position in the sequence
finite sequence
a function whose domain consists of a finite subset of the positive integers<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mo>{</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>…</mo><mi>n</mi><mo>}</mo><mtext> </mtext></mrow></annotation-xml></semantics></math>for some positive integer<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi></mrow></annotation-xml></semantics></math>
infinite sequence
a function whose domain is the set of positive integers
n factorial
the product of all the positive integers from 1 to<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><annotation-xml encoding="MathML-Content"><mrow><mtext> </mtext><mi>n</mi></mrow></annotation-xml></semantics></math>
nth term of a sequence
a formula for the general term of a sequence
recursive formula
a formula that defines each term of a sequence using previous term(s)
sequence
a function whose domain is a subset of the positive integers
term
a number in a sequence