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Inverse Functions
Think about your math class—if we assume that there are no siblings in your class then we can make the following statement: No two students in the class have the same set of parents.
This situation is similar to a one-to-one function.
Def: One-to-one functions:
A function within the domain A is called a one-to-one function if no two elements of A have the same image, that is,
f ( x 1 ) ≠ f ( x 2 ) whenever x 1 ≠ x 2
To understand this definition better, let’s go back to the situation in your math class. In this case, x 1 and x 2 would represent two students in your class, and f ( x 1 ) and f ( x 2 ) would represent the corresponding sets of parents. Obviously, if the two students are different people, then the corresponding sets of parents must be different also.
Another way to write this definition is the following:
If f ( x 1 ) = f ( x 2 ) then x 1 = x 2
Horizontal line test:
One-to-one functions are very easy to identify graphically.
Def: Horizontal line test:
A function is one-to-one if and only if no horizontal line intersects its graph more than once.
In other words, if you can draw at least one horizontal line anywhere on a graph, which intersects the graph more than once, then the graph is not one-to-one.
Consider the graph of a quadratic function. We already know that this is a parabola which opens upward or downward. You can find an infinite number of horizontal lines, which will intersect the graph more than once (twice in this case). Therefore, a quadratic function is not a one-to-one function.
Note: You may remember using the vertical line test previously in order to determine if a given graph is the graph of a function. The horizontal line test is different, and is used to determine if a given function is a one-to-one function.
Deciding whether a function is one-to-one:
It is very simple to tell whether a graph is the graph of a one-to-one function by using the horizontal line test. Now, we want to look at how to determine if a function is one-to-one just by looking at the function itself.
There is more than one way to prove that a function is or is not one-to-one. We will look at a couple of examples in order to help you begin to understand how to prove or determine whether a function is one-to-one.
Example 1: For the function f ( x ) = x^4 + 5, determine if the function is one-to-one.
Solution: Remember that there were two equivalent ways of stating the definition of a one-to-one function.
- f ( x 1 ) ≠ f ( x 2 ) whenever x 1 ≠ x 2
- If f ( x 1 ) = f ( x 2 ) then x 1 = x 2
So, we can use either statement. In this case, let’s use the first statement.
Let x 1 = –2 and x 1 = 2. Using these values for our variables, the resulting functions should not be equal if the function is one-to-one (by definition).
1 4
f x = f − = − + = + =
2 4
f x = f = + = + =
Since f ( x 1 ) = f ( x 2 ), then we do not have a one-to-one function.
Example 3:
(a) If f (3) = 15, find f -1^ (15).
If we compare this to the definition, we can see that x = 3 and y = 15. Therefore, f -1^ (15) = 3.
(b) If f (–3) = 7 find f -1^ (7).
By definition, f -1^ (7) = –3 since f (–3) = 7.
When working with the actual function (instead of specific values) it is necessary to follow a simple three-step process in order to determine the inverse of a function. We’ll look at an example to help us see this process.
Example 4: Find the inverse of the function f ( x ) = 5x + 7.
Step 1: Write y = f ( x ).
We substitute the expression in for f ( x ).
y = 5 x + 7
Step 2: Solve the resulting equation for x in terms of y (if possible).
x
y
y x
y x
so 5
y x
Step 3: Interchange x and y. The resulting equation is y = f –1( x ).
x y (Interchange x and y .)
Therefore, the inverse function is. 5
x f x
Example 5: Find the inverse of the function. 1
x
x f x
Step 1: 1
x
x y
Step 2: (Solve for x in terms of y. )
x y
y
y x y
y x xy
xy y x
y x x
x
x y
So y
y x −
Step 3: Interchanging x and y we have:
x
x y −
Therefore,. 2
x
x f x −
Checking your answers using the Inverse Function Property:
It is very helpful to note an important property of inverse functions.
Let f be a one-to-one function with domain A and range B. The inverse function f - satisfies the following cancellation properties.
f −^1 ( f ( x ))= x for every x in A f ( f −^1 ( x ))= x for every x in B
Conversely, any function f -1^ satisfying these equations is the inverse of f.
