# 6.1: Inverse Functions

Functions such as logarithms, exponential functions, and trigonometric functions are examples of ** transcendental functions.** If a function is transcendental, it cannot be expressed as a polynomial or rational function. That is, it is not an

*algebraic function.*In this chapter, we will begin by developing the concept of an inverse of a function and how it is linked to its original numerically, algebraically, and graphically. Later, we will take each type of elementary transcendental function—logarithmic, exponential, and trigonometric—individually and see the connection between them and their respective inverses, derivatives, and integrals.

## Learning Objectives

A student will be able to:

- Understand the basic properties of the inverse of a function and how to find it.
- Understand how a function and its inverse are represented graphically.
- Know the conditions of invertabilty of a function.

### One-to-One Functions

A function, as you know from your previous mathematics background, is a rule that assigns a single value in its range to each point in its domain. In other words, for each output number, there is one or more input numbers. However, a function never produces more than a single output for one input. A function is said to be a ** one-to-one** function if each output is associated with only one single input. For example, assigns the output for both and and thus it is not a

*one-to-one*function.

**One-to-One Function**

The function is one-to-one in a domain if whenever

There is an easy method to check if a function is one-to-one: draw a horizontal line across the graph. If the line intersects at only one point on the graph, then the function is one-to-one; otherwise, it is not. Notice in the figure below that the graph of is not one-to-one since the horizontal line intersects the graph more than once. But the function is a one-to-one function because the graph meets the horizontal line only once.

**Example 1:**

Determine whether the functions are one-to-one: (a) (b)

*Solution:*

It is best to graph both functions and draw on each a horizontal line. As you can see from the graphs, is not one-to-one since the horizontal line intersects it at two points. The function however, is indeed one-to-one since only one point is intersected by the horizontal line.

### The Inverse of a Function

We discussed above the condition for a one-to-one function: for each output, there is only one input. A one-to-one function can be reversed in such a way that the input of the function becomes the output and the output becomes an input. This reverse of the original function is called the *inverse* of the function. If is an inverse of a function then For example, the two functions and are inverses of each other since

Thus

and and are inverses of each other.

Note: In general,

**When is a function invertable?**

It is interesting to note that if a function is always increasing or always decreasing over its domain, then a horizontal line will cut through this graph at one point only. Then in this case is a one-to-one function and thus has an inverse. So if we can find a way to prove that a function is constantly increasing or decreasing, then it is ** invertable** or

**. From previous chapters, you have learned that if then must be increasing and if then must be decreasing.**

*monotonic*To summarize, a function has an inverse if it is one-to-one in its domain or if its derivative is either or

**Example 2:**

Given the polynomial function show that it is invertable (has an inverse).

*Solution:*

Taking the derivative, we find that for all We conclude that is one-to-one and invertable. Keep in mind that it may not be easy to find the inverse of (try it!), but we still know that it is indeed invertable.

**How to find the inverse of a one-to-one function:**

To find the inverse of a one-to-one function, simply solve for in terms of and then interchange and The resulting formula is the inverse

**Example 3:**

Find the inverse of .

*Solution:*

From the discussion above, we can find the inverse by first solving for in .

Interchanging ,

Replacing

which is the inverse of the original function .

### Graphs of Inverse Functions

What is the relationship between the graphs of and ? If the point is on the graph of then from the definition of the inverse, the point is on the graph of In other words, when we reverse the coordinates of a point on the graph of we automatically get a point on the graph of We conclude that and are ** reflections** of one another about the line That is, each is a mirror image of the other about the line The figure below shows an example of and, when the domain is restricted, its inverse and how they are reflected about .

It is important to note that for the function to have an inverse, we must restrict its domain to since that is the domain in which the function is increasing.

## Continuity and Differentiability of Inverse Functions

Since the graph of a one-to-one function and its inverse are reflections of one another about the line it would be safe to say that if the function has no breaks (no discontinuities) then will not have breaks either. This implies that if is continuous on the domain then its inverse is continuous on the range of For example, if , then its domain is and its range is This means that is continuous for all The inverse of is where its domain is all and its range is We conclude that if is a function with domain and range and it is continuous and one-to-one on then its inverse is continuous and one-to-one on the range of

Suppose that has a domain and a range If is differentiable and one-to-one on then its inverse is differentiable at any value in for which and

The formula above can be written in a form that is easier to remember:

In addition, if on its domain is either or then has an inverse function and is differentiable at all values of in the range of In this case, is given by the formula above. The example below illustrate this important theorem.

**Example 4:**

In Example 3, we were given the polynomial function and we showed that it is invertable. Show that it is differentiable and find the derivative of its inverse.

**Solution:**

Since for all is differentiable at all values of To find the derivative of if we let then

So

and

Since we are unable to solve for in terms of we leave the answer above in terms of Another way of solving the problem is to use Implicit Differentiation:

Since

differentiating implicitly,

Solving for we finally obtain

which is the same result.

## Review Questions

In problems #1 - 3, find the inverse function of and verify that

In problems #4 - 6, use the horizontal line test to verify whether the following functions have inverse.

In problems #7 - 8, use the functions and to find the specified functions.

In problems #9 - 10, show that is monotonic (invertable) on the given interval (and therefore has an inverse.)

## Review Answers

- Function has an inverse.
- Function does not have an inverse.
- Function does not have an inverse.
- on
- which is negative on the interval in question, so is monotonically decreasing.