\( \newcommand{\abs}[1]{\left| \, {#1} \, \right| } \) \( \newcommand{\cm}{\mathrm{cm} } \) \( \newcommand{\sec}{ \, \mathrm{sec} \, } \) \( \newcommand{\units}[1]{\,\text{#1}} \) \( \newcommand{\vhat}[1]{\,\hat{#1}} \) \( \newcommand{\vhati}{\,\hat{i}} \) \( \newcommand{\vhatj}{\,\hat{j}} \) \( \newcommand{\vhatk}{\,\hat{k}} \) \( \newcommand{\vect}[1]{\boldsymbol{\vec{#1}}} \) \( \newcommand{\norm}[1]{\|{#1}\|} \) \( \newcommand{\arccot}{ \, \mathrm{arccot} \, } \) \( \newcommand{\arcsec}{ \, \mathrm{arcsec} \, } \) \( \newcommand{\arccsc}{ \, \mathrm{arccsc} \, } \) \( \newcommand{\sech}{ \, \mathrm{sech} \, } \) \( \newcommand{\csch}{ \, \mathrm{csch} \, } \) \( \newcommand{\arcsinh}{ \, \mathrm{arcsinh} \, } \) \( \newcommand{\arccosh}{ \, \mathrm{arccosh} \, } \) \( \newcommand{\arctanh}{ \, \mathrm{arctanh} \, } \) \( \newcommand{\arccoth}{ \, \mathrm{arccoth} \, } \) \( \newcommand{\arcsech}{ \, \mathrm{arcsech} \, } \) \( \newcommand{\arccsch}{ \, \mathrm{arccsch} \, } \)

17Calculus Derivatives - Inverse Functions

17Calculus
Single Variable Calculus
Derivatives
Integrals
Multi-Variable Calculus
Precalculus
Functions

There are times when we need to know the derivative of an inverse function but it is not possible to calculate the actual inverse function.
For a full lecture on this topic, we recommend this video.

Prof Leonard - Calculus 2 Lecture 6.2: Derivatives of Inverse Functions [44mins-8secs]

video by Prof Leonard

When we are not able to calculate the inverse of a function but we need to know the derivative of the inverse at a point, we can use implicit differentiation to come up with a formula to do the calculation.

We need to be careful though to make sure that the function actually has an inverse. In order for a function to have an inverse, it must be one-to-one. One way to make sure it is one-to-one is to check that the function is always either increasing or decreasing in the interval we are concerned about. Of course, using calculus, we can take the derivative and find the intervals where the function is increasing and decreasing. Let's look at an example.

Example 1

built with GeoGebra

Does the function \( f(x) = x^3 + 7x + 2 \) have an inverse?
Let's take the derivative and see if we can determine anything from it.
\( f'(x) = 3x^2 + 7 \)
Notice that the derivative is always positive for all x since \(3x^2\) is positive and adding a positive 7 keeps the result positive. So, since the function is always increasing, it is one-to-one on it's entire domain and therefore has an inverse. To help you visualise this, here is the graph.

So, what do we do when have a function, we know it has an inverse but we can't find the inverse in order to take the derivative? Well, we can use implicit differentiation to find an expression for the derivative at a point as follows.

\( \begin{array}{rcl} y & = & f(x) \\ f^{-1}(y) & = & f^{-1}(f(x)) \\ f^{-1}(y) & = & x \\ \displaystyle{ \frac{d}{dx} } [ f^{-1}(y) ] & = & \displaystyle{ \frac{d}{dx} } [x] \\ (f^{-1}(y))' \displaystyle{ \frac{dy}{dx} } & = & 1 \\ (f^{-1}(y))' & = & \displaystyle{ \frac{1}{dy/dx} } \\ (f^{-1}(y))' & = & \displaystyle{ \frac{1}{f'(x)} } \\ \end{array} \)

Now we have an expression for the derivative of an inverse function at the point \((x_1,y_1)\) on \(f(x)\) which is the point \((y_1,x_1)\) on \(f^{-1}(x)\). \[ (f^{-1}(y_1))' = \frac{1}{f'(x_1)} \] The tick marks used to write the derivatives above hide important details of this equation. To more clearly show what is going on, we can write the equation like this. \[ \frac{d}{dy} [f^{-1}(y)] = \frac{1}{dy/dx} \] Notice that the derivative on the left is with respect to y and the derivative on the right is with respect to x.

This may still be a bit confusing, so let's look at it another way. If we have a function \(f(x)\) with an inverse \(g(x)\), then \[ g'(x) = \frac{1}{f'(g(x))} \] at the point \((x,g(x))\).

So how do we use all these equations to solve a problem? Let's go through an example to demonstrate how to keep everything straight.

Example 2

For the function \(f(x)=x^2; x \gt 0\), calculate \((f^{-1}(9))'\).
For this example, it is possible to find the inverse for \(x \gt 0 \) and we will do that later to check our answer. But let's use the above equations to get an answer first. We will build a table to keep track of everything. The first row shows information about \(f(x)\) and the second row shows information about the inverse of \(f(x)\).

\( f(x) = x^2 \)

\( f'(x) = 2x \)

\( f'(g(9)) \)

\(f^{-1}(x) = g(x)\)

\( (x, g(x) ) = (9,3)\)

\( g'(9) \)

We have a partially filled in table above with some basic information. We are given \(f(x)\) and the point \(x=9\) on the inverse function. We changed the notation a bit and wrote \(f^{-1}(x) = g(x)\). Using \(g(x)\) makes the notation simpler and easier to use. In the above table we took the derivative of \(f(x)\) and calculated the point \((9,3)\). To get the 3, we note that we are given \(x=9\) on \(g(x)\) and we know that since \(f(x)\) and \(g(x)\) are inverses, then \(f(x) = 9\). This means that \(x^2=9 \to x=\pm 3\) and since the problem stated that \(x \gt 0\), we know that \(x=3\). So that gives us the point \((9,3)\).

Now we can calculate \( f'(g(x)) \). From the table, \(g(9)=3\), so \(f'(g(9)) = f'(3)\) giving us \(f'(g(9)) = 6\). So we will fill in that spot in the table and the result in the last row and last column, which comes from \( g'(9) = 1/f'(g(9)) \).

\( f(x) = x^2 \)

\( f'(x) = 2x \)

\( f'(g(9)) = 6 \)

\( g(x) \)

\( (x, g(x) ) = (9,3)\)

\( g'(9) = 1/6 \)

Let's check our answer by directly calculating the derivative of the inverse function.
\( \begin{array}{rcl} g(x) & = & \sqrt{x} \\ g'(x) & = & \displaystyle{ \frac{1}{2x^{1/2}} } \\ g'(9) & = & \displaystyle{ \frac{1}{(2)9^{1/2}} = \frac{1}{6} } \text{✔} \end{array} \)

Final Answer

\( (f^{-1}(9))' = g'(9) = 1/6 \)

The key to working these problems correctly is to keep track of the x and y values and knowing what to use when. We think that the using a table, like we did in the second example above, helps.

Before you try some on your own, watch this quick video clip with a simplified explanation.

Michael Griffis - Calculus 1-The Derivative of an Inverse Function [4mins-34secs]

video by Michael Griffis

Okay, time for some practice problems.

You Can Have an Amazing Memory: Learn Life-Changing Techniques and Tips from the Memory Maestro

Practice

If \( f(x)=x^3+e^x \) and \( f^{-1}(1)=0 \), calculate the derivative of \(f^{-1}\) at \(x=1\). Make sure you check that this function has an inverse.

Problem Statement

If \( f(x)=x^3+e^x \) and \( f^{-1}(1)=0 \), calculate the derivative of \(f^{-1}\) at \(x=1\). Make sure you check that this function has an inverse.

Solution

PatrickJMT - 2349 video solution

video by PatrickJMT

Log in to rate this practice problem and to see it's current rating.

If \( f(x) = \cos x + 3x \) and \( f^{-1}(3\pi/2) = \pi/2 \), calculate the derivative of \( f^{-1} \) at \( x=3\pi/2 \). Make sure to check that \( f(x) \) has an inverse.

Problem Statement

If \( f(x) = \cos x + 3x \) and \( f^{-1}(3\pi/2) = \pi/2 \), calculate the derivative of \( f^{-1} \) at \( x=3\pi/2 \). Make sure to check that \( f(x) \) has an inverse.

Solution

PatrickJMT - 2350 video solution

video by PatrickJMT

Log in to rate this practice problem and to see it's current rating.

For \( f(x)=2x^3+3x^2+7x+4 \), calculate \( (f^{-1})'(4) \).

Problem Statement

For \( f(x)=2x^3+3x^2+7x+4 \), calculate \( (f^{-1})'(4) \).

Solution

VinTeachesMath - 2351 video solution

video by VinTeachesMath

Log in to rate this practice problem and to see it's current rating.

For \( f(x)=x^3+2x-1\), calculate \( (f^{-1})'(2) \).

Problem Statement

For \( f(x)=x^3+2x-1\), calculate \( (f^{-1})'(2) \).

Solution

Michael Griffis - 2352 video solution

video by Michael Griffis

Log in to rate this practice problem and to see it's current rating.

For \(\displaystyle{ f(x)=\frac{1}{27}(x^5+2x^3) }\), calculate \( (f^{-1})'(-11) \).

Problem Statement

For \(\displaystyle{ f(x)=\frac{1}{27}(x^5+2x^3) }\), calculate \( (f^{-1})'(-11) \).

Solution

Michael Griffis - 2353 video solution

video by Michael Griffis

Log in to rate this practice problem and to see it's current rating.

For \( f(x) = x^3-4/x \), calculate \( (f^{-1})'(6) \).

Problem Statement

For \( f(x) = x^3-4/x \), calculate \( (f^{-1})'(6) \).

Solution

Michael Griffis - 2354 video solution

video by Michael Griffis

Log in to rate this practice problem and to see it's current rating.

Really UNDERSTAND Calculus

Log in to rate this page and to see it's current rating.

Topics You Need To Understand For This Page

Related Topics and Links

To bookmark this page and practice problems, log in to your account or set up a free account.

Search Practice Problems

Do you have a practice problem number but do not know on which page it is found? If so, enter the number below and click 'page' to go to the page on which it is found or click 'practice' to be taken to the practice problem.

memorize to learn

Kindle Unlimited Membership Plans

As an Amazon Associate I earn from qualifying purchases.

I recently started a Patreon account to help defray the expenses associated with this site. To keep this site free, please consider supporting me.

Support 17Calculus on Patreon

Practice Search

Do NOT follow this link or you will be banned from the site!

When using the material on this site, check with your instructor to see what they require. Their requirements come first, so make sure your notation and work follow their specifications.

DISCLAIMER - 17Calculus owners and contributors are not responsible for how the material, videos, practice problems, exams, links or anything on this site are used or how they affect the grades or projects of any individual or organization. We have worked, to the best of our ability, to ensure accurate and correct information on each page and solutions to practice problems and exams. However, we do not guarantee 100% accuracy. It is each individual's responsibility to verify correctness and to determine what different instructors and organizations expect. How each person chooses to use the material on this site is up to that person as well as the responsibility for how it impacts grades, projects and understanding of calculus, math or any other subject. In short, use this site wisely by questioning and verifying everything. If you see something that is incorrect, contact us right away so that we can correct it.

Links and banners on this page are affiliate links. We carefully choose only the affiliates that we think will help you learn. Clicking on them and making purchases help you support 17Calculus at no extra charge to you. However, only you can decide what will actually help you learn. So think carefully about what you need and purchase only what you think will help you.

We use cookies on this site to enhance your learning experience.

17calculus

Copyright © 2010-2022 17Calculus, All Rights Reserved     [Privacy Policy]     [Support]     [About]

mathjax.org
Real Time Web Analytics