4.4 Root Functions and Their Transformations

Learning Objectives

In this section, you will:

  • Identify characteristic of odd and even root functions.
  • Determine the properties of transformed root functions.

A root function is a power function of the form [latex]f\left(x\right)=x^\frac{1}{n}[/latex], where [latex]n[/latex] is a positive integer greater than one.  For example, [latex]f\left(x\right)=x^\frac{1}{2}=\sqrt[\leftroot{1}\uproot{2} ]{x}[/latex] is the square-root function and  [latex]g\left(x\right)=x^\frac{1}{3}=\sqrt[\leftroot{1}\uproot{2}3]{x}[/latex] is the cube-root functions.

The root functions [latex]f\left(x\right)=x^\frac{1}{n}[/latex] have defining characteristics depending on whether [latex]n[/latex] is odd or even.  For all positive even integers [latex]n\geq2[/latex], the domain of  [latex]f\left(x\right)=x^\frac{1}{n}[/latex] is the interval [latex]\left[0,\infty\right).[/latex]  Figure 1 shows the the functions [latex]f\left(x\right)=x^\frac{1}{2}=\sqrt[\leftroot{1}\uproot{2} ]{x},[/latex]  [latex]g\left(x\right)=x^\frac{1}{4}=\sqrt[\leftroot{1}\uproot{2}4]{x}[/latex] and [latex]h\left(x\right)=x^\frac{1}{6}=\sqrt[\leftroot{1}\uproot{2}6]{x}[/latex] which are all even root functions.  

Graphic comparing even root functions

Figure 1

Notice that these graphs have similar shapes, very much like that of the square root function in the toolkit. However, as the value of n increases, the graphs steepen somewhat near the origin and become flatter away from the origin growing more slowly.  The [latex]x[/latex] and [latex]y[/latex] intercepts of these functions are [latex]\left(0,0\right)[/latex]. The end behavior for the even root function only makes sense as [latex]x[/latex] increases without bound since negative values are not in the domain.  We observe as [latex]x\to\infty,\textrm{ }f\left(x\right)\to\infty[/latex].

For all positive odd integers [latex]n\geq3[/latex], the domain of  [latex]f\left(x\right)=x^\frac{1}{n}[/latex] is the set of all real numbers.  Since [latex]x^\frac{1}{n}=\left(-x\right)^\frac{1}{n}[/latex] for positive odd integers [latex]n[/latex], [latex]f\left(x\right)=x^\frac{1}{n}[/latex] is an odd function if [latex]n[/latex] is a positive odd number. Figure 2 shows the functions [latex]f\left(x\right)=x^\frac{1}{3}=\sqrt[\leftroot{1}\uproot{2}3]{x},[/latex] [latex]g\left(x\right)=x^\frac{1}{5}=\sqrt[\leftroot{1}\uproot{2}5]{x}[/latex] and [latex]h\left(x\right)=x^\frac{1}{7}=\sqrt[\leftroot{1}\uproot{2}7]{x}[/latex] which are all odd root functions.

Graphic compares odd root functions.

Figure 2

Notice that these graphs look similar to the cube root function in the toolkit. Again, as the value of n increases, the graphs steepens near the origin and become flatter away from the origin increasing more slowly.  The [latex]x[/latex] and [latex]y[/latex] intercepts of these functions are [latex]\left(0,0\right)[/latex].  The end behavior for the even root function is expressed as [latex]x\to\infty,\textrm{ }f\left(x\right)\to\infty[/latex] for large values of [latex]x[/latex] and as [latex]x\to-\infty,\textrm{ }f\left(x\right)\to-\infty[/latex] for very negative values of [latex]x.[/latex]

Transformations of Root Functions

For transformations of even root functions, the domain and range are effected by horizontal and vertical shifts, reflections and stretches.  There are two methods you can use to find the domain.  The first method is to use algebra and the idea that even root functions must have non-negative values under the root symbol.  The expression under the root symbol is set greater than or equal to zero and the inequality is solved to find the domain.  Alternatively, you can use the properties of the transformation by identifying the basic function and determining where the point (0,0) gets transformed to in the new function.  The x-coordinate will be the starting or ending point for the domain.  If there is not a horizontal reflection, the domain will be from that value to the right and if there is a horizontal reflection, then the domain will go from that value to the left.

The range is determined by identifying the basic function and determining what transformation is applied to get the function you are working with.  After applying transformation to the point (0,0), the y-coordinate tells you where the range starts or ends.  If there is not a vertical reflection the range will be from that value to infinity and if there is a vertical reflection the range will be from minus infinity to that value.

How To

Given a root function, find the domain and range.

Domain Method 1: Algebraically

  1. Set the expression under the root symbol greater than or equal to zero and solve.
  2. Write the solution in interval notation.  Remember to use the square bracket as appropriate.

Domain Method 2: Transformations

  1. Identify the basic root function.
  2. Describe the transformation in words and then determine where the point (0,0) gets mapped to under that transformation.
  3. If there is not a vertical reflection, the domain is from the x-coordinate of the transformed point to infinity.  If there is a vertical reflection, the domain is from minus infinity to that x-coordinate.

Range

  1. Identify the basic root function.
  2. Describe the transformation in words and then determine where the point (0,0) gets mapped to under that transformation.
  3. The y-coordinate tells you where the range starts or ends.  If there is not a vertical reflection the range will be from that value to infinity and if there is a vertical reflection the range will be from minus infinity to that value.

Example 1:  The Domain and Range of an Even Root Function

Find the domain, range and intercepts of the square root function shifted 3 units left and 1 unit up.

 

Example 2:  Domain and Intercepts of Even Root Functions

Find the domain and range for

a. [latex]g\left(x\right)=\sqrt[\leftroot{1}\uproot{2}4]{3-2x}.[/latex]

b. [latex]h\left(x\right)=-3\sqrt[\leftroot{1}\uproot{2}4]{x}.[/latex]

Intercepts of Even Root Functions

Transformations of even root functions may or may not have [latex]x[/latex] or [latex]y[/latex] intercepts.  If [latex]x = 0[/latex] is in the domain of the transformed function then there will be a y-intercept found by evaluating [latex]f\left(0\right).[/latex]  If [latex]y=0[/latex] is in the range, then there will be an x-intercept and we solve [latex]f\left(x\right)=0.[/latex]

Example 3:  Intercepts of Transformations of Even Root Functions

Find x-intercepts and y-intercepts for

a. [latex]g\left(x\right)=\sqrt[\leftroot{1}\uproot{2}4]{3-2x}.[/latex]

b. [latex]h\left(x\right)=-3\sqrt[\leftroot{1}\uproot{2}4]{x}.[/latex]

c. [latex]f\left(x\right)=\sqrt[\leftroot{1}\uproot{2} ]{x+3}+1.[/latex]

Try It #1

Find the domain, range and intercepts of the fourth root function shifted 2 units right and 1 unit down.

Try It #2

Find the domain, x-intercepts and y-intercepts for [latex]g\left(x\right)=\sqrt[\leftroot{1}\uproot{2} ]{7-0.5x}.[/latex]

End Behavior of Even Root  Functions

The final property to examine for even root functions and their transformations is the end or long term behavior.  Since the domain is only part of the real numbers only behavior to the left or right needs to be determined depending on whether the domain goes toward minus infinity or plus infinity.

Example 4:  End Behavior of a Horizontally Reflected Even Root Function

Determine the end behavior of [latex]k\left(x\right)=\sqrt[\leftroot{1}\uproot{2}6]{2-x}.[/latex]

Properties of Odd Root Functions

Odd root functions do not have their domains and ranges change under transformations since they are defined on [latex]\left(-\infty,\infty\right).[/latex]  However with horizontal and vertical shifts, the intercepts are expected to change and if there is a horizontal or vertical reflection, the end behavior may be effected.

Example 5:  Properties of a Reflected Odd Root Function

Determine the domain, range, x-intercept, y-intercept and end behavior of the function [latex]f\left(x\right)=\sqrt[\leftroot{1}\uproot{2}3]{3-x}+1.[/latex]

Key Concepts

  • Root functions are steep near the origin and then grow slowly.
  • The domain, range, intercepts and end behavior may change when even root functions are transformed.
    • Intercepts may not exist for all transformed even root functions.
    • Only one side of end behavior makes sense for transformed even root functions.
  • The domain and range for transformed odd root functions remains [latex]\left(-\infty,\infty\right)[/latex]
  • The intercepts and end behavior may change when odd root functions are transformed.  However, there will and [latex]x[/latex] and [latex]y[/latex] intercept and the end behavior must be considered on both the right and left.