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1.4: Fractions

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Learning Objectives

By the end of this section, you will be able to:

  • Simplify fractions
  • Multiply and divide fractions
  • Add and subtract fractions
  • Use the order of operations to simplify fractions
  • Evaluate variable expressions with fractions

A more thorough introduction to the topics covered in this section can be found in the Elementary Algebra chapter, Foundations.

Simplify Fractions

A fraction is a way to represent parts of a whole. The fraction 23 represents two of three equal parts (Figure 1.4.1). In the fraction 23, the 2 is called the numerator and the 3 is called the denominator. The line is called the fraction bar.

Figure shows a circle divided in three equal parts. 2 of these are shaded.
Figure 1.4.1: In the circle, 23 of the circle is shaded—2 of the 3 equal parts.

FRACTION

A fraction is written ab, where b0 and

a is the numerator and b is the denominator.

A fraction represents parts of a whole. The denominator b is the number of equal parts the whole has been divided into, and the numerator a indicates how many parts are included.

Fractions that have the same value are equivalent fractions. The Equivalent Fractions

Property allows us to find equivalent fractions and also simplify fractions.

If a, b, and c are numbers where b0,c0,

then ab=a·cb·c and a·cb·c=ab.

A fraction is considered simplified if there are no common factors, other than 1, in its numerator and denominator.

For example,

23 is simplified because there are no common factors of 2 and 3.

1015 is not simplified because 5 is a common factor of 10 and 15.

We simplify, or reduce, a fraction by removing the common factors of the numerator and denominator. A fraction is not simplified until all common factors have been removed. If an expression has fractions, it is not completely simplified until the fractions are simplified.

Sometimes it may not be easy to find common factors of the numerator and denominator. When this happens, a good idea is to factor the numerator and the denominator into prime numbers. Then divide out the common factors using the Equivalent Fractions Property.

Simplify 315770.

Answer

Step 1 is to rewrite the numerator and denominator to show the common factors. If needed, use a factor tree. Here, we rewrite 315 and 770 as the product of the primes. Starting with minus 315 divided by 770, we get, minus 3 times 3 time 5 times 7 divided by 2 times 5 times 7 times 11. Step 2 is to simplify using the equivalent fractions property by dividing out the common factors.  Here we mark the common factors of 5 and 7 and then canceling them we obtain negative 3 times 3 over quantity 2 times 11. Step 3 is to multiply the remaining factors, if necessary. We get minus 9 by 22.

Simplify 69120.

Answer

2340

EXAMPLE 1.4.3

Simplify 120192.

Answer

58

We now summarize the steps you should follow to simplify fractions.

SIMPLIFY A FRACTION.
  1. Rewrite the numerator and denominator to show the common factors.
    If needed, factor the numerator and denominator into prime numbers first.
  2. Simplify using the Equivalent Fractions Property by dividing out common factors.
  3. Multiply any remaining factors.

Multiply and Divide Fractions

Many people find multiplying and dividing fractions easier than adding and subtracting fractions.

To multiply fractions, we multiply the numerators and multiply the denominators.

FRACTION MULTIPLICATION

If a, b, c, and d are numbers where b0, and d0, then

ab·cd=acbd

To multiply fractions, multiply the numerators and multiply the denominators.

When multiplying fractions, the properties of positive and negative numbers still apply, of course. It is a good idea to determine the sign of the product as the first step. In Example, we will multiply negative and a positive, so the product will be negative.

When multiplying a fraction by an integer, it may be helpful to write the integer as a fraction. Any integer, a, can be written as a1. So, for example, 3=31.

Exercise 1.4.4

Multiply: 125(20x).

Answer

The first step is to find the sign of the product. Since the signs are the same, the product is positive.

  alt

Determine the sign of the product. The signs are the same, so the product is positive.

alt
Write 20x as a fraction. alt
Multiply. alt

Rewrite 20 to show the common factor 5 and divide it out.

alt
Simplify. alt
Exercise 1.4.5

Multiply: 113(9a).

Answer

33a

Exercise 1.4.6

Multiply: 137(14b).

Answer

26b

Now that we know how to multiply fractions, we are almost ready to divide. Before we can do that, we need some vocabulary. The reciprocal of a fraction is found by inverting the fraction, placing the numerator in the denominator and the denominator in the numerator. The reciprocal of 23 is 32. Since 4 is written in fraction form as 41, the reciprocal of 4 is 14.

To divide fractions, we multiply the first fraction by the reciprocal of the second.

FRACTION DIVISION

If a, b, c, and d are numbers where b0,c0, and d0, then

ab÷cd=abdc

To divide fractions, we multiply the first fraction by the reciprocal of the second.

We need to say b0, c0, and d0, to be sure we don’t divide by zero!

Exercise 1.4.7

Find the quotient: 718÷(1427).

Answer
  718÷(1427)

To divide, multiply the first fraction by the reciprocal of the second.

alt

Determine the sign of the product, and then multiply.

alt
Rewrite showing common factors. alt
Remove common factors. alt
Simplify. alt

Divide: 727÷(3536).

Answer

415

Exercise 1.4.9

Divide: 514÷(1528).

Answer

23

The numerators or denominators of some fractions contain fractions themselves. A fraction in which the numerator or the denominator is a fraction is called a complex fraction.

Definition: COMPLEX FRACTION

A complex fraction is a fraction in which the numerator or the denominator contains a fraction.

Some examples of complex fractions are:

6733458x256

To simplify a complex fraction, remember that the fraction bar means division. For example, the complex fraction 3458 means 34÷58.

Exercise 1.4.10

Simplify: x2xy6.

Answer

x2xy6Rewrite as division.x2÷xy6Multiply the first fraction by the reciprocal of the second.x2·6xyMultiply.x·62·xyLook for common factors.x·3·22·x·yDivide common factors and simplify.3y

Simplify: a8ab6.

Answer

34b

Simplify: p2pq8.

Answer

4q

Add and Subtract Fractions

When we multiplied fractions, we just multiplied the numerators and multiplied the denominators right straight across. To add or subtract fractions, they must have a common denominator.

FRACTION ADDITION AND SUBTRACTION

If a, b, and c are numbers where c0, then

ac+bc=a+bc and acbc=abc

To add or subtract fractions, add or subtract the numerators and place the result over the common denominator.

The least common denominator (LCD) of two fractions is the smallest number that can be used as a common denominator of the fractions. The LCD of the two fractions is the least common multiple (LCM) of their denominators.

LEAST COMMON DENOMINATOR

The least common denominator (LCD) of two fractions is the least common multiple (LCM) of their denominators.

After we find the least common denominator of two fractions, we convert the fractions to equivalent fractions with the LCD. Putting these steps together allows us to add and subtract fractions because their denominators will be the same!

EXAMPLE 1.4.13: How to Add or Subtract Fractions

Add: 712+518.

Answer

The expression is 7 by 12 plus 5 by 18. Step 1 is to check if the two numbers have a common denominator. Since they do not, rewrite each fraction with the LCD (least common denominator). For finding the LCD, we write the factors of 12 as 2 times 2 times 2 and the factors of 18 as 2 times 3 times 3. The LCD is 2 times 2 times 3 times 3, which is equal to 36. Step 2 is to add or subtract the fractions.  Here we add, obtaining 31 over 36. Step 3 is to simplify is possible. Since 31 is prime, its only factors are 1and 31. Since 31 does not go into 36, the answer is simplified.

EXAMPLE 1.4.14

Add: 712+1115.

Answer

7960

EXAMPLE 1.4.15

Add: 1315+1720.

Answer

10360

ADD OR SUBTRACT FRACTIONS.
  1. Do they have a common denominator?
    • Yes—go to step 2.
    • No—rewrite each fraction with the LCD (least common denominator).
      • Find the LCD.
      • Change each fraction into an equivalent fraction with the LCD as its denominator.
  2. Add or subtract the fractions.
  3. Simplify, if possible.

We now have all four operations for fractions. Table summarizes fraction operations.

Fraction Multiplication Fraction Division
abcd=acbd ab÷cd=abdc
Multiply the numerators and multiply the denominators Multiply the first fraction by the reciprocal of the second.
Fraction Addition Fraction Subtraction
ac+bc=a+bc acbc=abc
Add the numerators and place the sum over the common denominator. Subtract the numerators and place the difference over the common denominator.

To multiply or divide fractions, an LCD is NOT needed.

To add or subtract fractions, an LCD is needed.

When starting an exercise, always identify the operation and then recall the methods needed for that operation.

Simplify: ⓐ 5x63105x6·310.

Answer

First ask, “What is the operation?” Identifying the operation will determine whether or not we need a common denominator. Remember, we need a common denominator to add or subtract, but not to multiply or divide.

What is the operation? The operation is subtraction.Do the fractions have a common denominator? No.5x6310Find the LCD of 6 and 10The LCD is 30.6=2·310_=2·5_LCD=2·3·5LCD=30Rewrite each fraction as an equivalent fraction with the LCD.5x·56·53·310·325x30930Subtract the numerators and place thedifference over the common denominators.25x930Simplify,if possible.There are no common factors.The fraction is simplified.

What is the operation? Multiplication.25x6·310To multiply fractions,multiply the numeratorsand multiply the denominators.25x·36·10Rewrite, showing common factors.Remove common factors.5x·32·3·2·5Simplify.x4

Notice, we needed an LCD to add 25x6310, but not to multiply 25x6310.

EXAMPLE 1.4.17

Simplify: ⓐ 3a4893a4·89.

Answer

27a32362a3

EXAMPLE 1.4.18

Simplify: ⓐ4k5164k516.

Answer

24k5302k15

Use the Order of Operations to Simplify Fractions

The fraction bar in a fraction acts as grouping symbol. The order of operations then tells us to simplify the numerator and then the denominator. Then we divide.

SIMPLIFY AN EXPRESSION WITH A FRACTION BAR.
  1. Simplify the expression in the numerator. Simplify the expression in the denominator.
  2. Simplify the fraction.

Where does the negative sign go in a fraction? Usually the negative sign is in front of the fraction, but you will sometimes see a fraction with a negative numerator, or sometimes with a negative denominator. Remember that fractions represent division. When the numerator and denominator have different signs, the quotient is negative.

13=13negativepositive=negative

13=13positivenegative=negative

PLACEMENT OF NEGATIVE SIGN IN A FRACTION

For any positive numbers a and b,

ab=ab=ab

EXAMPLE 1.4.19

Simplify: 4(3)+6(2)3(2)2.

Answer

The fraction bar acts like a grouping symbol. So completely simplify the numerator and the denominator separately.

4(3)+6(2)3(2)2Multiply.12+(12)62Simplify.248Divide.3

EXAMPLE 1.4.20

Simplify:8(2)+4(3)5(2)+3.

Answer

4

EXAMPLE 1.4.21

Simplify: 7(1)+9(3)5(3)2.

Answer

2

Now we’ll look at complex fractions where the numerator or denominator contains an expression that can be simplified. So we first must completely simplify the numerator and denominator separately using the order of operations. Then we divide the numerator by the denominator as the fraction bar means division.

EXAMPLE 1.4.22: How to Simplify Complex Fractions

Simplify: (12)24+32.

Answer

The expression is 1 by 2 the whole squared divided by 4 plus 3 squared. Step 1 is to simplify the numerator, which becomes 1 by 4.
Step 2 is to simplify the denominator.  Adding 4 and 9 gives us 13 in the denominator.
Step 3 is to divide the numerator by the denominator and simplify if possible. Now the expression becomes 1 by 4 divided by 13 by 1, which equals 1 by 4 multiplied by 1 by 13, which equals 1 by 52

EXAMPLE 1.4.23

Simplify: (13)223+2.

Answer

190

EXAMPLE 1.4.24

Simplify: 1+42(14)2.

Answer

272

SIMPLIFY COMPLEX FRACTIONS.
  1. Simplify the numerator.
  2. Simplify the denominator.
  3. Divide the numerator by the denominator. Simplify if possible.
EXAMPLE 1.4.25

Simplify: 12+233416.

Answer

It may help to put parentheses around the numerator and the denominator.

12+233416Simplify the numerator (LCD=6) and simplify the denominator (LCD=12).(36+46)(912212)Simplify.(76)(712)Divide the numerator by the denominator.76÷712Simplify.76127Divide out common factors.762671Simplify.2

EXAMPLE 1.4.26

Simplify: 13+123413.

Answer

2

EXAMPLE 1.4.27

Simplify: 231214+13.

Answer

27

Evaluate Variable Expressions with Fractions

We have evaluated expressions before, but now we can evaluate expressions with fractions. Remember, to evaluate an expression, we substitute the value of the variable into the expression and then simplify.

EXAMPLE 1.4.28

Evaluate 2x2y when x=14 and y=23.

Answer

Substitute the values into the expression.

  alt
alt alt
Simplify exponents first. alt
Multiply; divide out the common factors. Notice we write 16 as 2⋅2⋅42·2·4 to make it easy to remove common factors. alt
Simplify. alt
EXAMPLE 1.4.29

Evaluate 3ab2 when a=23 and b=12.

Answer

12

EXAMPLE 1.4.30

Evaluate 4c3d when c=12 and d=43.

Answer

23

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

  • Adding Fractions with Unlike Denominators

Key Concepts

  • If a, b, and c are numbers where b0,c0, then

ab=a·cb·c and a·cb·c=ab.

  • How to simplify a fraction.
    1. Rewrite the numerator and denominator to show the common factors.
      If needed, factor the numerator and denominator into prime numbers first.
    2. Simplify using the Equivalent Fractions Property by dividing out common factors.
    3. Multiply any remaining factors.
  • If a, b, c, and d are numbers where b0, and d0, then

    ab·cd=acbd

    To multiply fractions, multiply the numerators and multiply the denominators.

  • If a, b, c, and d are numbers where b0,c0, and d0, then

    ab÷cd=abdc

    To divide fractions, we multiply the first fraction by the reciprocal of the second.

  • If a, b, and c are numbers where c0, then

    ac+bc=a+bc and acbc=abc

    To add or subtract fractions, add or subtract the numerators and place the result over the common denominator.

  • How to add or subtract fractions.
    1. Do they have a common denominator?
      • Yes—go to step 2.
      • No—rewrite each fraction with the LCD (least common denominator).
        • Find the LCD.
        • Change each fraction into an equivalent fraction with the LCD as its denominator.
    2. Add or subtract the fractions.
    3. Simplify, if possible.
  • How to simplify an expression with a fraction bar.
    1. Simplify the expression in the numerator. Simplify the expression in the denominator.
    2. Simplify the fraction.
  • For any positive numbers a and b,

    ab=ab=ab

  • How to simplify complex fractions.
    1. Simplify the numerator.
    2. Simplify the denominator.
    3. Divide the numerator by the denominator. Simplify if possible.

Glossary

complex fraction
A fraction in which the numerator or the denominator is a fraction is called a complex fraction.
denominator
In a fraction, written ab, where b0, the denominator b is the number of equal parts the whole has been divided into.
equivalent fractions
Equivalent fractions are fractions that have the same value.
fraction
A fraction is written ab, where b0, and a is the numerator and b is the denominator. A fraction represents parts of a whole.
least common denominator
The least common denominator (LCD) of two fractions is the least common multiple (LCM) of their denominators.
numerator
In a fraction, written ab, where b0, the numerator a indicates how many parts are included.
reciprocal
The reciprocal of a fraction is found by inverting the fraction, placing the numerator in the denominator and the denominator in the numerator.

This page titled 1.4: Fractions is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.

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