Monday, April 27, 2015

Algebra: Inverse Functions, Finding Inverses

I'm so sorry this post is a week late. Life got busy. :-)

Now that we understand the definition of an inverse function and how to test them, we can learn how to find them. In this post, I'll share my favorite method of finding the inverse. How do you like to do it?

Step 1: Rewrite with the variables y and x. Use y in place of f(x), g(x), etc.

This is straight-forward. If you are given a function in terms of different variables, such as f(b) and b, change them to y and x. Use y for the dependent variable (usually f(x) or g(x), etc.) and x for the independent variable (usually already x, but may be another letter).

Step 2: Swap the independent and dependent variables.

Swap x and y. Shown below.

Step 3: Solve for y.

Using this new equation, solve for y. Shown below.

Step 4: Replace y with the inverse notation of the original function.

In this case, we replace y with f^(-1)(x) because the original function was f(x). If the original function were, say, g(b), then we would replace x with b, and y with g^(-1)(b). Shown below.

Step 5: Test the inverse.

Using the method we discussed in the previous post, test the inverse you found. Those steps are shown below.

Our answer passes the test of the theorem we learned about, so it is correct.


Let's try another example. This one is a little more complicated, and will show you why it is important to test your answer at the end!

Steps 1-3: Rewrite in terms of y and x; swap the variables; solve for y.

All three of the first steps are shown below. Notice the last step of solving for y. When you take the square root of a variable, you must include both the positive and negative outside of the square root. Therefore, we actually have two answers for the inverse, this time.

Step 4: Rewrite with inverse notation of the original function.

Step 5: Test the inverse(s).

We must test both of our answers. First, we will test the positive version, shown below.

This answer passes the test. Now, we must test the negative version, shown below.

This answer passes the first portion of the test, but fails the second portion. Remember, both parts of the theorem must hold true in order for the two functions to be considered inverses. Therefore, g^(-1)(x) = - sqrt(x-1) is NOT an inverse of g(x) = x^2 + 1! We simply throw out this answer and only give the positive version, shown below.


That's how I like to find the inverse of a function. :-) If you have another method you prefer, I'd love to hear all about it in a comment!

That concludes this algebra series on inverse functions. If you have a request for my next series, leave it as a comment on this, or any, post, or send it to me via e-mail (find my e-mail on the Contact page). I'd love to hear from you!