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In the last example
from the previous section we looked at the two functions 
and and saw that
and as noted in that section this means that there is a nice
relationship between these two functions.
Let’s see just what that relationship is. Consider the following evaluations.
In the first case we plugged into and got a value of -5. We then turned around and plugged into and got a value of -1, the number that we
started off with.
In the second case we did something similar. Here we plugged into and got a value of ,
we turned around and plugged this into and got a value of 2, which is again the
number that we started with.
Note that we really are doing some function composition
here. The first case is really,
and the second case is really,
Note as well that these both agree with the formula for the
compositions that we found in the previous section. We get back out of the function evaluation
the number that we originally plugged into the composition.
So, just what is going on here? In some way we can think of these two
functions as undoing what the other did to a number. In the first case we plugged into and then plugged the result from this function
evaluation back into and in some way undid what had done to and gave us back the original x that we started with.
Function pairs that exhibit this behavior are called inverse functions. Before formally defining inverse functions
and the notation that we’re going to use for them we need to get a definition
out of the way.
A function is called one-to-one
if no two values of x produce the
same y. Mathematically this is the same as saying,
So, a function is one-to-one if whenever we plug different
values into the function we get different function values.
Sometimes it is easier to understand this definition if we
see a function that isn’t one-to-one.
Let’s take a look at a function that isn’t one-to-one. The function is not one-to-one because both and . In other words there are two different values
of x that produce the same value of y.
Note that we can turn into a one-to-one function if we restrict
ourselves to . This can sometimes be done with functions.
Showing that a function is one-to-one is often tedious
and/or difficult. For the most part we are
going to assume that the functions that we’re going to be dealing with in this
course are either one-to-one or we have restricted the domain of the function
to get it to be a one-to-one function.
Now, let’s formally define just what inverse functions are. Given two one-to-one functions and if
then we say that and are inverses
of each other. More specifically we will
say that is the inverse
of and denote it by
Likewise we could also say that is the inverse
of and denote it by
The notation that we use really depends upon the
problem. In most cases either is
acceptable.
For the two functions that we started off this section with
we could write either of the following two sets of notation.
Now, be careful with the notation for inverses. The “-1” is NOT an exponent despite the fact
that is sure does look like one! When
dealing with inverse functions we’ve got to remember that
This is one of the more common mistakes that students make
when first studying inverse functions.
The process for finding the inverse of a function is a
fairly simple one although there are a couple of steps that can on occasion be somewhat
messy. Here is the process
Finding the Inverse
of a Function
That’s the process.
Most of the steps are not all that bad but as mentioned in the process
there are a couple of steps that we really need to be careful with since it is
easy to make mistakes in those steps.
In the verification step we technically really do need to
check that both and are true.
For all the functions that we are going to be looking at in this course
if one is true then the other will also be true. However, there are functions (they are beyond
the scope of this course however) for which it is possible for only one of
these to be true. This is brought up
because in all the problems here we will be just checking one of them. We just need to always remember that
technically we should check both.
Let’s work some examples.
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Example 1 Given
find .
Solution
Now, we already know what the inverse to this function is
as we’ve already done some work with it.
However, it would be nice to actually start with this since we know
what we should get. This will work as
a nice verification of the process.
So, let’s get started.
We’ll first replace with y.
Next, replace all x’s
with y and all y’s with x.
Now, solve for y.
Finally replace y
with .
Now, we need to verify the results. We already took care of this in the
previous section, however, we really should follow the process so we’ll do
that here. It doesn’t matter which of
the two that we check we just need to check one of them. This time we’ll check that is true.
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