3.3: Power Functions and Polynomial Functions
Identifying Power Functions
In order to better understand the bird problem, we need to understand a specific type of function. A power function is a function with a single term that is the product of a real number, a coefficient, and a variable raised to a fixed real number. (A number that multiplies a variable raised to an exponent is known as a coefficient.)
As an example, consider functions for area or volume. The function for the area of a circle with radius \(r\) is
\[A(r)={\pi}r^2 \nonumber\]
and the function for the volume of a sphere with radius \(r\) is
\[V(r)=\dfrac{4}{3}{\pi}r^3 \nonumber\]
Both of these are examples of power functions because they consist of a coefficient, \({\pi}\) or \(\dfrac{4}{3}{\pi}\), multiplied by a variable \(r\) raised to a power.
A power function is a function that can be represented in the form
\[f(x)=kx^p \label{power}\]
where \(k\) and \(p\) are real numbers, and \(k\) is known as the coefficient .
Identifying End Behavior of Power Functions
Figure \(\PageIndex{2}\) shows the graphs of \(f(x)=x^2\), \(g(x)=x^4\) and and \(h(x)=x^6\), which are all power functions with even, whole-number powers. Notice that these graphs have similar shapes, very much like that of the quadratic function in the toolkit. However, as the power increases, the graphs flatten somewhat near the origin and become steeper away from the origin.
To describe the behavior as numbers become larger and larger, we use the idea of infinity. We use the symbol \(\infty\) for positive infinity and \(−\infty\) for negative infinity. When we say that “x approaches infinity,” which can be symbolically written as \(x{\rightarrow}\infty\), we are describing a behavior; we are saying that \(x\) is increasing without bound.
With the even-power function, as the input increases or decreases without bound, the output values become very large, positive numbers. Equivalently, we could describe this behavior by saying that as \(x\) approaches positive or negative infinity, the \(f(x)\) values increase without bound. In symbolic form, we could write
\[\text{as } x{\rightarrow}{\pm}{\infty}, \;f(x){\rightarrow}{\infty} \nonumber\]
Figure \(\PageIndex{3}\) shows the graphs of \(f(x)=x^3\), \(g(x)=x^5\), and \(h(x)=x^7\), which are all power functions with odd, whole-number powers. Notice that these graphs look similar to the cubic function in the toolkit. Again, as the power increases, the graphs flatten near the origin and become steeper away from the origin.
These examples illustrate that functions of the form \(f(x)=x^n\) reveal symmetry of one kind or another. First, in Figure \(\PageIndex{2}\) we see that even functions of the form \(f(x)=x^n\), \(n\) even, are symmetric about the \(y\)-axis. In Figure \(\PageIndex{3}\) we see that odd functions of the form \(f(x)=x^n\), \(n\) odd, are symmetric about the origin.
For these odd power functions, as \(x\) approaches negative infinity, \(f(x)\) decreases without bound. As \(x\) approaches positive infinity, \(f(x)\) increases without bound. In symbolic form we write
\[\begin{align*} &\text{as }x{\rightarrow}-{\infty},\;f(x){\rightarrow}-{\infty} \\ &\text{as }x{\rightarrow}{\infty},\;f(x){\rightarrow}{\infty} \end{align*}\]
The behavior of the graph of a function as the input values get very small \((x{\rightarrow}−{\infty})\) and get very large \(x{\rightarrow}{\infty}\) is referred to as the end behavior of the function. We can use words or symbols to describe end behavior.
Figure \(\PageIndex{4}\) shows the end behavior of power functions in the form \(f(x)=kx^n\) where \(n\) is a non-negative integer depending on the power and the constant.
Identifying Polynomial Functions
An oil pipeline bursts in the Gulf of Mexico, causing an oil slick in a roughly circular shape. The slick is currently 24 miles in radius, but that radius is increasing by 8 miles each week. We want to write a formula for the area covered by the oil slick by combining two functions. The radius \(r\) of the spill depends on the number of weeks \(w\) that have passed. This relationship is linear.
\[r(w)=24+8w \nonumber\]
We can combine this with the formula for the area A of a circle.
\[A(r)={\pi}r^2 \nonumber\]
Composing these functions gives a formula for the area in terms of weeks.
\[ \begin{align*} A(w)&=A(r(w)) \\ &=A(24+8w) \\ & ={\pi}(24+8w)^2 \end{align*}\]
Multiplying gives the formula.
\[A(w)=576{\pi}+384{\pi}w+64{\pi}w^2 \nonumber\]
This formula is an example of a polynomial function. A polynomial function consists of either zero or the sum of a finite number of non-zero terms, each of which is a product of a number, called the coefficient of the term, and a variable raised to a non-negative integer power.
Let \(n\) be a non-negative integer. A polynomial function is a function that can be written in the form
\[f(x)=a_nx^n+...+a_2x^2+a_1x+a_0 \label{poly}\]
This is called the general form of a polynomial function. Each \(a_i\) is a coefficient and can be any real number. Each product \(a_ix^i\) is a term of a polynomial function .
Which of the following are polynomial functions?
- \(f(x)=2x^3⋅3x+4\)
- \(g(x)=−x(x^2−4)\)
- \(h(x)=5\sqrt{x}+2\)
Solution
The first two functions are examples of polynomial functions because they can be written in the form of Equation \ref{poly}, where the powers are non-negative integers and the coefficients are real numbers.
- \(f(x)\) can be written as \(f(x)=6x^4+4\).
- \(g(x)\) can be written as \(g(x)=−x^3+4x\).
- \(h(x)\) cannot be written in this form and is therefore not a polynomial function.
Identifying the Degree and Leading Coefficient of a Polynomial Function
Because of the form of a polynomial function, we can see an infinite variety in the number of terms and the power of the variable. Although the order of the terms in the polynomial function is not important for performing operations, we typically arrange the terms in descending order of power, or in general form. The degree of the polynomial is the highest power of the variable that occurs in the polynomial; it is the power of the first variable if the function is in general form. The leading term is the term containing the highest power of the variable, or the term with the highest degree. The leading coefficient is the coefficient of the leading term.
We often rearrange polynomials so that the powers are descending.
When a polynomial is written in this way, we say that it is in general form .
Identifying End Behavior of Polynomial Functions
Knowing the degree of a polynomial function is useful in helping us predict its end behavior. To determine its end behavior, look at the leading term of the polynomial function. Because the power of the leading term is the highest, that term will grow significantly faster than the other terms as \(x\) gets very large or very small, so its behavior will dominate the graph. For any polynomial, the end behavior of the polynomial will match the end behavior of the term of highest degree (Table \(\PageIndex{3}\)).
| Polynomial Function | Leading Term | Graph of Polynomial Function |
|---|---|---|
| \(f(x)=5x^4+2x^3−x−4\) | \(5x^4\) | |
| \(f(x)=−2x^6−x^5+3x^4+x^3\) | \(−2x^6\) | |
| \(f(x)=3x^5−4x^4+2x^2+1\) | \(3x^5\) | |
| \(f(x)=−6x^3+7x^2+3x+1\) | \(−6x^3\) |
Describe the end behavior and determine a possible degree of the polynomial function in Figure \(\PageIndex{8}\).
Solution
As the input values \(x\) get very large, the output values \(f(x)\) increase without bound. As the input values \(x\) get very small, the output values \(f(x)\) decrease without bound. We can describe the end behavior symbolically by writing
\[\text{as } x{\rightarrow}{\infty}, \; f(x){\rightarrow}{\infty} \nonumber\]
\[\text{as } x{\rightarrow}-{\infty}, \; f(x){\rightarrow}-{\infty} \nonumber\]
In words, we could say that as \(x\) values approach infinity, the function values approach infinity, and as \(x\) values approach negative infinity, the function values approach negative infinity.
We can tell this graph has the shape of an odd degree power function that has not been reflected, so the degree of the polynomial creating this graph must be odd and the leading coefficient must be positive.
Identifying Local Behavior of Polynomial Functions
In addition to the end behavior of polynomial functions, we are also interested in what happens in the “middle” of the function. In particular, we are interested in locations where graph behavior changes. A turning point is a point at which the function values change from increasing to decreasing or decreasing to increasing.
We are also interested in the intercepts. As with all functions, the \(y\)-intercept is the point at which the graph intersects the vertical axis. The point corresponds to the coordinate pair in which the input value is zero. Because a polynomial is a function, only one output value corresponds to each input value so there can be only one \(y\)-intercept \((0,a_0)\). The \(x\)-intercepts occur at the input values that correspond to an output value of zero. It is possible to have more than one \(x\)-intercept. See Figure \(\PageIndex{10}\).
A turning point of a graph is a point at which the graph changes direction from increasing to decreasing or decreasing to increasing. The \(y\)-intercept is the point at which the function has an input value of zero. The \(x\)-intercepts are the points at which the output value is zero.
Comparing Smooth and Continuous Graphs
The degree of a polynomial function helps us to determine the number of \(x\)-intercepts and the number of turning points. A polynomial function of \(n^\text{th}\) degree is the product of \(n\) factors, so it will have at most \(n\) roots or zeros, or \(x\)-intercepts. The graph of the polynomial function of degree \(n\) must have at most \(n–1\) turning points. This means the graph has at most one fewer turning point than the degree of the polynomial or one fewer than the number of factors.
A continuous function has no breaks in its graph: the graph can be drawn without lifting the pen from the paper. A smooth curve is a graph that has no sharp corners. The turning points of a smooth graph must always occur at rounded curves. The graphs of polynomial functions are both continuous and smooth.
A polynomial of degree \(n\) will have, at most, \(n\) \(x\)-intercepts and \(n−1\) turning points.