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Study Guides > College Algebra

Identify 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 rr is
[latex]A \left(r\right)=\pi {r}^{2}\[/latex]
and the function for the volume of a sphere with radius rr is
[latex]V \left(r\right)=\frac{4}{3}\pi {r}^{3}\[/latex]
Both of these are examples of power functions because they consist of a coefficient, π\pi or 43π\frac{4}{3}\pi , multiplied by a variable rr raised to a power.

A General Note: Power Function

A power function is a function that can be represented in the form
f(x)=kxpf\left(x\right)=k{x}^{p}
where k and p are real numbers, and k is known as the coefficient.

Q & A

Is f(x)=2xf\left(x\right)={2}^{x} a power function? No. A power function contains a variable base raised to a fixed power. This function has a constant base raised to a variable power. This is called an exponential function, not a power function.

Example 1: Identifying Power Functions

Which of the following functions are power functions?

begin{cases}f\left(x\right)=1hfill & text{Constant function}hfill \ f\left(x\right)=xhfill & text{Identify function}hfill \ f\left(x\right)={x}^{2}hfill & text{Quadratic}text{ }text{ function}hfill \ f\left(x\right)={x}^{3}hfill & text{Cubic function}hfill \ f\left(x\right)=\frac{1}{x} hfill & text{Reciprocal function}hfill \ f\left(x\right)=\frac{1}{{x}^{2}}hfill & text{Reciprocal squared function}hfill \ f\left(x\right)=sqrt{x}hfill & text{Square root function}hfill \ f\left(x\right)=sqrt[3]{x}hfill & text{Cube root function}hfill end{cases}

Solution

All of the listed functions are power functions. The constant and identity functions are power functions because they can be written as f(x)=x0f\left(x\right)={x}^{0} and f(x)=x1f\left(x\right)={x}^{1} respectively. The quadratic and cubic functions are power functions with whole number powers f(x)=x2f\left(x\right)={x}^{2} and f(x)=x3f\left(x\right)={x}^{3}. The reciprocal and reciprocal squared functions are power functions with negative whole number powers because they can be written as f(x)=x1f\left(x\right)={x}^{-1} and f(x)=x2f\left(x\right)={x}^{-2}. The square and cube root functions are power functions with \fractional powers because they can be written as f(x)=x1/2f\left(x\right)={x}^{1/2} or f(x)=x1/3f\left(x\right)={x}^{1/3}.

Try It 1

Which functions are power functions? f(x)=2x24x3f\left(x\right)=2{x}^{2}\cdot4{x}^{3} g(x)=x5+5x34xg\left(x\right)=-{x}^{5}+5{x}^{3}-4x h(x)=2x513x2+4h\left(x\right)=\frac{2{x}^{5}-1}{3{x}^{2}+4} Solution

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