## Inverse Functions

### Learning Outcomes

• Verify inverse functions.
• Determine the domain and range of an inverse function, and restrict the domain of a function to make it one-to-one.
• Find or evaluate the inverse of a function.
• Use the graph of a one-to-one function to graph its inverse function on the same axes.

A reversible heat pump is a climate-control system that is an air conditioner and a heater in a single device. Operated in one direction, it pumps heat out of a house to provide cooling. Operating in reverse, it pumps heat into the building from the outside, even in cool weather, to provide heating. As a heater, a heat pump is several times more efficient than conventional electrical resistance heating.

If some physical machines can run in two directions, we might ask whether some of the function “machines” we have been studying can also run backwards. Figure 1 provides a visual representation of this question. In this section, we will consider the reverse nature of functions.

Figure 1. Can a function “machine” operate in reverse?

## Characteristics of Inverse Functions

Suppose a fashion designer traveling to Milan for a fashion show wants to know what the temperature will be. He is not familiar with the Celsius scale. To get an idea of how temperature measurements are related, he asks his assistant, Betty, to convert 75 degrees Fahrenheit to degrees Celsius. She finds the formula $C=\frac{5}{9}\left(F - 32\right)$ and substitutes 75 for $F$ to calculate $\frac{5}{9}\left(75 - 32\right)\approx {24}^{ \circ} {C}$.

Knowing that a comfortable 75 degrees Fahrenheit is about 24 degrees Celsius, he sends his assistant the week’s weather forecast for Milan, and asks her to convert all of the temperatures to degrees Fahrenheit.

At first, Betty considers using the formula she has already found to complete the conversions. After all, she knows her algebra, and can easily solve the equation for $F$ after substituting a value for $C$. For example, to convert 26 degrees Celsius, she could write

\begin{align}&26=\frac{5}{9}\left(F - 32\right) \\[1.5mm] &26\cdot \frac{9}{5}=F - 32 \\[1.5mm] &F=26\cdot \frac{9}{5}+32\approx 79 \end{align}

After considering this option for a moment, however, she realizes that solving the equation for each of the temperatures will be awfully tedious. She realizes that since evaluation is easier than solving, it would be much more convenient to have a different formula, one that takes the Celsius temperature and outputs the Fahrenheit temperature.

The formula for which Betty is searching corresponds to the idea of an inverse function, which is a function for which the input of the original function becomes the output of the inverse function and the output of the original function becomes the input of the inverse function.

Given a function $f\left(x\right)$, we represent its inverse as ${f}^{-1}\left(x\right)$, read as “$f$ inverse of $x$.” The raised $-1$ is part of the notation. It is not an exponent; it does not imply a power of $-1$ . In other words, ${f}^{-1}\left(x\right)$ does not mean $\frac{1}{f\left(x\right)}$ because $\frac{1}{f\left(x\right)}$ is the reciprocal of $f$ and not the inverse.

The “exponent-like” notation comes from an analogy between function composition and multiplication: just as ${a}^{-1}a=1$ (1 is the identity element for multiplication) for any nonzero number $a$, so ${f}^{-1}\circ f$ equals the identity function, that is,

$\left({f}^{-1}\circ f\right)\left(x\right)={f}^{-1}\left(f\left(x\right)\right)={f}^{-1}\left(y\right)=x$

This holds for all $x$ in the domain of $f$. Informally, this means that inverse functions “undo” each other. However, just as zero does not have a reciprocal, some functions do not have inverses.

Given a function $f\left(x\right)$, we can verify whether some other function $g\left(x\right)$ is the inverse of $f\left(x\right)$ by checking whether either $g\left(f\left(x\right)\right)=x$ or $f\left(g\left(x\right)\right)=x$ is true. We can test whichever equation is more convenient to work with because they are logically equivalent (that is, if one is true, then so is the other.)

For example, $y=4x$ and $y=\frac{1}{4}x$ are inverse functions.

$\left({f}^{-1}\circ f\right)\left(x\right)={f}^{-1}\left(4x\right)=\frac{1}{4}\left(4x\right)=x$

and

$\left({f}^{}\circ {f}^{-1}\right)\left(x\right)=f\left(\frac{1}{4}x\right)=4\left(\frac{1}{4}x\right)=x$

A few coordinate pairs from the graph of the function $y=4x$ are (−2, −8), (0, 0), and (2, 8). A few coordinate pairs from the graph of the function $y=\frac{1}{4}x$ are (−8, −2), (0, 0), and (8, 2). If we interchange the input and output of each coordinate pair of a function, the interchanged coordinate pairs would appear on the graph of the inverse function.

### A General Note: Inverse Function

For any one-to-one function $f\left(x\right)=y$, a function ${f}^{-1}\left(x\right)$ is an inverse function of $f$ if ${f}^{-1}\left(y\right)=x$. This can also be written as ${f}^{-1}\left(f\left(x\right)\right)=x$ for all $x$ in the domain of $f$. It also follows that $f\left({f}^{-1}\left(x\right)\right)=x$ for all $x$ in the domain of ${f}^{-1}$ if ${f}^{-1}$ is the inverse of $f$.

The notation ${f}^{-1}$ is read “$f$ inverse.” Like any other function, we can use any variable name as the input for ${f}^{-1}$, so we will often write ${f}^{-1}\left(x\right)$, which we read as $f$ inverse of $x$“.

Keep in mind that ${f}^{-1}\left(x\right)\ne \frac{1}{f\left(x\right)}$ and not all functions have inverses.

### Example: Identifying an Inverse Function for a Given Input-Output Pair

If for a particular one-to-one function $f\left(2\right)=4$ and $f\left(5\right)=12$, what are the corresponding input and output values for the inverse function?

### Try It

Given that ${h}^{-1}\left(6\right)=2$, what are the corresponding input and output values of the original function $h?$

### How To: Given two functions $f\left(x\right)$ and $g\left(x\right)$, test whether the functions are inverses of each other.

1. Determine whether $f\left(g\left(x\right)\right)=x$ and $g\left(f\left(x\right)\right)=x$.
2. If both statements are true, then $g={f}^{-1}$ and $f={g}^{-1}$. If either statement is false, then $g\ne {f}^{-1}$ and $f\ne {g}^{-1}$.

### Example: Testing Inverse Relationships Algebraically

If $f\left(x\right)=\dfrac{1}{x+2}$ and $g\left(x\right)=\dfrac{1}{x}-2$, is $g={f}^{-1}?$

### Try It

If $f\left(x\right)={x}^{3}-4$ and $g\left(x\right)=\sqrt[3]{x+4}$, is $g={f}^{-1}?$

### Example: Determining Inverse Relationships for Power Functions

If $f\left(x\right)={x}^{3}$ (the cube function) and $g\left(x\right)=\frac{1}{3}x$, is $g={f}^{-1}?$

### Try It

If $f\left(x\right)={\left(x - 1\right)}^{3}\text{and}g\left(x\right)=\sqrt[3]{x}+1$, is $g={f}^{-1}?$

## Determine the Domain and Range of an Inverse Function

The outputs of the function $f$ are the inputs to ${f}^{-1}$, so the range of $f$ is also the domain of ${f}^{-1}$. Likewise, because the inputs to $f$ are the outputs of ${f}^{-1}$, the domain of $f$ is the range of ${f}^{-1}$. We can visualize the situation.

Domain and range of a function and its inverse

When a function has no inverse function, it is possible to create a new function where that new function on a limited domain does have an inverse function. For example, the inverse of $f\left(x\right)=\sqrt{x}$ is ${f}^{-1}\left(x\right)={x}^{2}$, because a square “undoes” a square root; but the square is only the inverse of the square root on the domain $\left[0,\infty \right)$, since that is the range of $f\left(x\right)=\sqrt{x}$.

We can look at this problem from the other side, starting with the square (toolkit quadratic) function $f\left(x\right)={x}^{2}$. If we want to construct an inverse to this function, we run into a problem, because for every given output of the quadratic function, there are two corresponding inputs (except when the input is 0). For example, the output 9 from the quadratic function corresponds to the inputs 3 and –3. But an output from a function is an input to its inverse; if this inverse input corresponds to more than one inverse output (input of the original function), then the “inverse” is not a function at all! To put it differently, the quadratic function is not a one-to-one function; it fails the horizontal line test, so it does not have an inverse function. In order for a function to have an inverse, it must be a one-to-one function.

In many cases, if a function is not one-to-one, we can still restrict the function to a part of its domain on which it is one-to-one. For example, we can make a restricted version of the square function $f\left(x\right)={x}^{2}$ with its range limited to $\left[0,\infty \right)$, which is a one-to-one function (it passes the horizontal line test) and which has an inverse (the square-root function).

If $f\left(x\right)={\left(x - 1\right)}^{2}$ on $\left[1,\infty \right)$, then the inverse function is ${f}^{-1}\left(x\right)=\sqrt{x}+1$.

• The domain of $f$ = range of ${f}^{-1}$ = $\left[1,\infty \right)$.
• The domain of ${f}^{-1}$ = range of $f$ = $\left[0,\infty \right)$.

Q & A

Is it possible for a function to have more than one inverse?

No. If two supposedly different functions, say, $g$ and $h$, both meet the definition of being inverses of another function $f$, then you can prove that $g=h$. We have just seen that some functions only have inverses if we restrict the domain of the original function. In these cases, there may be more than one way to restrict the domain, leading to different inverses. However, on any one domain, the original function still has only one unique inverse.

### A General Note: Domain and Range of Inverse Functions

The range of a function $f\left(x\right)$ is the domain of the inverse function ${f}^{-1}\left(x\right)$.

The domain of $f\left(x\right)$ is the range of ${f}^{-1}\left(x\right)$.

### How To: Given a function, find the domain and range of its inverse.

1. If the function is one-to-one, write the range of the original function as the domain of the inverse, and write the domain of the original function as the range of the inverse.
2. If the domain of the original function needs to be restricted to make it one-to-one, then this restricted domain becomes the range of the inverse function.

### Example: Finding the Inverses of Toolkit Functions

Identify which of the toolkit functions besides the quadratic function are not one-to-one, and find a restricted domain on which each function is one-to-one, if any. The toolkit functions are reviewed below. We restrict the domain in such a fashion that the function assumes all y-values exactly once.

 Constant Identity Quadratic Cubic Reciprocal $f\left(x\right)=c$ $f\left(x\right)=x$ $f\left(x\right)={x}^{2}$ $f\left(x\right)={x}^{3}$ $f\left(x\right)=\frac{1}{x}$ Reciprocal squared Cube root Square root Absolute value $f\left(x\right)=\frac{1}{{x}^{2}}$ $f\left(x\right)=\sqrt[3]{x}$ $f\left(x\right)=\sqrt{x}$ $f\left(x\right)=|x|$

### Try It

The domain of the function $f$ is $\left(1,\infty \right)$ and the range of the function $f$ is $\left(\mathrm{-\infty },-2\right)$. Find the domain and range of the inverse function.

## Define and Graph an Inverse

Once we have a one-to-one function, we can evaluate its inverse at specific inverse function inputs or construct a complete representation of the inverse function in many cases.

## Inverting Tabular Functions

Suppose we want to find the inverse of a function represented in table form. Remember that the domain of a function is the range of the inverse and the range of the function is the domain of the inverse. So we need to interchange the domain and range.

Each row (or column) of inputs becomes the row (or column) of outputs for the inverse function. Similarly, each row (or column) of outputs becomes the row (or column) of inputs for the inverse function.

### Example: Interpreting the Inverse of a Tabular Function

A function $f\left(t\right)$ is given below, showing distance in miles that a car has traveled in $t$ minutes. Find and interpret ${f}^{-1}\left(70\right)$.

 $t\text{ (minutes)}$ 30 50 70 90 $f\left(t\right)\text{ (miles)}$ 20 40 60 70

### Try It

Using the table below, find and interpret (a) $\text{ }f\left(60\right)$, and (b) $\text{ }{f}^{-1}\left(60\right)$.

 $t\text{ (minutes)}$ 30 50 60 70 90 $f\left(t\right)\text{ (miles)}$ 20 40 50 60 70

## Evaluating the Inverse of a Function, Given a Graph of the Original Function

The domain of a function can be read by observing the horizontal extent of its graph. We find the domain of the inverse function by observing the vertical extent of the graph of the original function, because this corresponds to the horizontal extent of the inverse function. Similarly, we find the range of the inverse function by observing the horizontal extent of the graph of the original function, as this is the vertical extent of the inverse function. If we want to evaluate an inverse function, we find its input within its domain, which is all or part of the vertical axis of the original function’s graph.

### How To: Given the graph of a function, evaluate its inverse at specific points.

1. Find the desired input of the inverse function on the $y$-axis of the given graph.
2. Read the inverse function’s output from the $x$-axis of the given graph.

### Example: Evaluating a Function and Its Inverse from a Graph at Specific Points

A function $g\left(x\right)$ is given below. Find $g\left(3\right)$ and ${g}^{-1}\left(3\right)$.

### Try It

Using the graph in the previous example, (a) find ${g}^{-1}\left(1\right)$, and (b) estimate ${g}^{-1}\left(4\right)$.

## Finding Inverses of Functions Represented by Formulas

Sometimes we will need to know an inverse function for all elements of its domain, not just a few. If the original function is given as a formula—for example, $y$ as a function of $x-$ we can often find the inverse function by solving to obtain $x$ as a function of $y$.

### How To: Given a function represented by a formula, find the inverse.

1. Verify that $f$ is a one-to-one function.
2. Replace $f\left(x\right)$ with $y$.
3. Interchange $x$ and $y$.
4. Solve for $y$, and rename the function ${f}^{-1}\left(x\right)$.

### Example: Inverting the Fahrenheit-to-Celsius Function

Find a formula for the inverse function that gives Fahrenheit temperature as a function of Celsius temperature.

$C=\frac{5}{9}\left(F - 32\right)$

### Try It

Solve for $x$ in terms of $y$ given $y=\frac{1}{3}\left(x - 5\right)$

### Example: Solving to Find an Inverse Function

Find the inverse of the function $f\left(x\right)=\dfrac{2}{x - 3}+4$.

### Example: Solving to Find an Inverse with Radicals

Find the inverse of the function $f\left(x\right)=2+\sqrt{x - 4}$.

### Try It

What is the inverse of the function $f\left(x\right)=2-\sqrt{x}$? State the domains of both the function and the inverse function.
Use an online graphing tool to graph the function, its inverse, and $f(x) = x$ to check whether you are correct.

## Graph a Function’s Inverse

Now that we can find the inverse of a function, we will explore the graphs of functions and their inverses. Let us return to the quadratic function $f\left(x\right)={x}^{2}$ restricted to the domain $\left[0,\infty \right)$, on which this function is one-to-one, and graph it as below.

Quadratic function with domain restricted to [0, ∞).

Restricting the domain to $\left[0,\infty \right)$ makes the function one-to-one (it will obviously pass the horizontal line test), so it has an inverse on this restricted domain.

We already know that the inverse of the toolkit quadratic function is the square root function, that is, ${f}^{-1}\left(x\right)=\sqrt{x}$. What happens if we graph both $f\text{ }$ and ${f}^{-1}$ on the same set of axes, using the $x\text{-}$ axis for the input to both $f\text{ and }{f}^{-1}?$

We notice a distinct relationship: The graph of ${f}^{-1}\left(x\right)$ is the graph of $f\left(x\right)$ reflected about the diagonal line $y=x$, which we will call the identity line, shown below.

Square and square-root functions on the non-negative domain

This relationship will be observed for all one-to-one functions, because it is a result of the function and its inverse swapping inputs and outputs. This is equivalent to interchanging the roles of the vertical and horizontal axes.

### Example: Finding the Inverse of a Function Using Reflection about the Identity Line

Given the graph of $f\left(x\right)$, sketch a graph of ${f}^{-1}\left(x\right)$.

Q & A

Is there any function that is equal to its own inverse?

Yes. If $f={f}^{-1}$, then $f\left(f\left(x\right)\right)=x$, and we can think of several functions that have this property. The identity function does, and so does the reciprocal function, because

$\frac{1}{\frac{1}{x}}=x$

Any function $f\left(x\right)=c-x$, where $c$ is a constant, is also equal to its own inverse.

## Key Concepts

• If $g\left(x\right)$ is the inverse of $f\left(x\right)$, then $g\left(f\left(x\right)\right)=f\left(g\left(x\right)\right)=x$.
• Each of the toolkit functions, except $y=c$ has an inverse. Some need a restricted domain.
• For a function to have an inverse, it must be one-to-one (pass the horizontal line test).
• A function that is not one-to-one over its entire domain may be one-to-one on part of its domain.
• For a tabular function, exchange the input and output rows to obtain the inverse.
• The inverse of a function can be determined at specific points on its graph.
• To find the inverse of a function $y=f\left(x\right)$, switch the variables $x$ and $y$. Then solve for $y$ as a function of $x$.
• The graph of an inverse function is the reflection of the graph of the original function across the line $y=x$.

## Glossary

inverse function
for any one-to-one function $f\left(x\right)$, the inverse is a function ${f}^{-1}\left(x\right)$ such that ${f}^{-1}\left(f\left(x\right)\right)=x$ for all $x$ in the domain of $f$; this also implies that $f\left({f}^{-1}\left(x\right)\right)=x$ for all $x$ in the domain of ${f}^{-1}$