Describing Inverses of Functions

The inverse of a function can itself be a function. In this section, properties of inverses and invertible functions will be explored.

Concept

Inverse of a Function

The inverse of a function reverses its $x$ - $y$ coordinates. If, for a function $f,$ $x$ is an input and $y$ is its corresponding output, for the inverse, $f^{\text{-} 1}, y$ is the input and $x$ would be the corresponding output. $f(x)=y \quad \Leftrightarrow \quad f^{\text{-} 1}(y)=x$

Some function families are inverses of each other. This is because some functions undo each other. For example,

x^2

and

\pm\sqrt{x}

are inverses because radicals and exponents (with the same index) undo each other.

Method

Finding the Inverse of a Function

Depending on how a function is presented, finding its inverse can be done in different ways. When a function's rule is given, finding the inverse algebraically is advantageous. Consider the function

f(x)= \dfrac{2x-1}{3}.

1

Replace

f(x)

with

y

To begin, since $f(x)=y$ describes the input-output relationship of the function, replace $f(x)$ with $y$ in the function rule. $f(x)= \dfrac{2x-1}{3} \quad \Leftrightarrow \quad y= \dfrac{2x-1}{3}$

2

Switch

x

and

y

Because the inverse of a function reverses $x$ and $y,$ the variables can be switched. Notice that every other piece in the function rule remains the same. ${\color{#009600}{y}}= \dfrac{2 {\color{#0000FF}{x}}-1}{3} \quad \Rightarrow {\color{#0000FF}{x}}= \dfrac{2 {\color{#009600}{y}}-1}{3}$

3

Solve for

y

Solve the resulting equation from Step 2 for

y.

Here, this will involve using the inverse operations.

x=\dfrac{2y-1}{3}

MultEqn

\text{LHS} \cdot 3=\text{RHS}\cdot 3

3x=2y-1

AddEqn

\text{LHS}+1=\text{RHS}+1

3x+1=2y

DivEqn

\left.\text{LHS}\middle/2\right.=\left.\text{RHS}\middle/2\right.

\dfrac{3x+1}{2}=y

RearrangeEqnRearrange equation

y=\dfrac{3x+1}{2}

4

Replace

y

with

f^{\text{-} 1}(x)

Just as $f(x)=y$ shows the input-output relationship of $f,$ so does $f^{\text{-} 1}(x)=y.$ Thus, replacing $y$ with $f^{\text{-} 1}(x)$ gives the rule for the inverse of $f.$

$y=\dfrac{3x+1}{2} \quad \Leftrightarrow \quad f^{\text{-} 1}=\dfrac{3x+1}{2}$

Notice that in $f,$ the input is multiplied by $2,$ decreased by $1$ and divided by $3.$ From the rule of $f^{\text{-} 1},$ it can be seen that $x$ undergoes the inverse of these operation in the reverse order. Specifically, $x$ is multiplied by $3,$ increased by $1,$ and divided by $2.$

Determine the inverse of the function

fullscreen

Exercise

Some of the coordinates of the function $g$ are shown in the table. Find $g^{\text{-} 1},$ then graph $g$ and $g^{\text{-} 1}$ on the same coordinate plane.

$x$	$g(x)$
$\text{-} 4$	$3$
$\text{-} 2$	$2$
$0$	$1$
$2$	$0$
$4$	$\text{-} 1$

Show Solution

Solution

An inverse of a function reverses its $x$ - and $y$ -coordinates. When a function is expressed as a table of values, finding its inverse means switching the coordinates. For example, the point $(\text{-}4,3)$ on $g$ becomes $(3,\text{-} 4)$ on $g^{\text{-} 1}.$ The following table describes $g^{\text{-} 1}(x).$

$x$	$g^{\text{-}1}(x)$
$3$	$\text{-}4$
$2$	$\text{-}2$
$1$	$0$
$0$	$2$
$\text{-}1$	$4$

We can graph both $g$ and $g^{\text{-} 1}$ by marking the points from both tables on the same coordinate plane.

Concept

Graphs of Functions and Inverses

The graphs of a function and its inverse have a noteworthy relationship. In the coordinate plane, the function $f(x)=2x-3 \text{ and its inverse } f^{\text{-}1}(x)=\frac{x+3}{2}$ are graphed. Notice that the points of $f^{\text{-} 1}$ are the reversed points of $f.$

Because the coordinates of the points are reversed, $f^{\text{-}1}(x)$ is a reflection of $f(x)$ in the line $y=x.$

This is true for all functions and their inverses.

Concept

Invertible Function

A function is said to be invertible if its inverse is also a function. An example of an invertible function is $f(x)=0.5x-2,$ because its inverse, $f^{\text{-} 1}(x)=2x+4,$ is a linear function. Consider the function $g(x)=x^2,$ whose inverse is $\pm\sqrt{x}.$

Notice that the inverse fails the Vertical Line Test. Thus, it is not a function, which, in turn means that $g$ is not invertible. However, if the domain of $g$ is restricted to $x\geq 0,$ the inverse, $g^{\text{-} 1}(x)=\sqrt{x},$ is a function.

Thus, some functions that are not inherently invertible can be made invertible by restricting their domains.

Method

Horizontal Line Test

The Vertical Line Test determines if the graph of a relation is a function, or if each $x$ -value corresponds with exactly one $y$ -value. For a function to be invertible, $\begin{aligned} \text{each } y & \text{-value} \\ \text{must cor} & \text{respond to} \\ \text{exactly o} & \text{ne } x\text{-value.} \end{aligned}$ This leads to the Horizontal Line Test, which is performed by moving an imaginary horizontal line across the graph of a function. If this line intersects the graph more than once anywhere, the function is not invertible.

Perform test

Change function

1

2

3

4

Example

Determine the inverse of the function