1. Functions and Their Inverses
9-5
Functions and Their Inverses
Warm Up
Lesson Presentation
Lesson Quiz
Holt Algebra2

2. Warm Up
Solve for x in terms of y. 1. 2. 3.
4. y = 2ln x

3. Objectives Determine whether the inverse of a function is a function.
Write rules for the inverses of functions.

4. Vocabulary
one-to-one function

5. In Lesson 7-2, you learned that the inverse of a function f(x) “undoes” f(x). Its graph is a reflection across line y = x. The inverse may or not be a function.
Recall that the vertical-line test (Lesson 1-6) can help you determine whether a relation is a function. Similarly, the horizontal-line test can help you determine whether the inverse of a function is a function.

7. Example 1A: Using the Horizontal-Line Test
Use the horizontal-line test to determine whether the inverse of the blue relation is a function.
The inverse is a function because no horizontal line passes through two points on the graph.

8. Example 1B: Using the Horizontal-Line Test
Use the horizontal-line test to determine whether the inverse of the red relation is a function.
The inverse is a not a function because a horizontal line passes through more than one point on the graph.

9. Check It Out! Example 1
Use the horizontal-line test to determine whether the inverse of each relation is a function.
The inverse is a function because no horizontal line passes through two points on the graph.

10. Recall from Lesson 7-2 that to write the rule for the inverse of a function, you can exchange x and y and solve the equation for y. Because the value of x and y are switched, the domain of the function will be the range of its inverse and vice versa.

11. Example 2: Writing Rules for inverses
Find the inverse of Determine whether it is a function, and state its domain and range.
Step 1 The horizontal-line test shows that the inverse is a function. Note that the domain and range of f are all real numbers.

12. Example 2 Continued
Step 1 Find the inverse.
Rewrite the function using y instead of f(x).
Switch x and y in the equation.
Cube both sides.
Simplify.
Isolate y.

13. Example 2 Continued
Because the inverse is a function,
The domain of the inverse is the range of f(x):{x|x R}.
The range is the domain of f(x):{y|y R}.
Check Graph both relations to see that they are symmetric about y = x.

14. Check It Out! Example 2
Find the inverse of f(x) = x3 – 2. Determine whether it is a function, and state its domain and range.
Step 1 The horizontal-line test shows that the inverse is a function. Note that the domain and range of f are all real numbers.

15. Check It Out! Example 2 Continued
Step 1 Find the inverse.
y = x3 – 2
Rewrite the function using y instead of f(x).
x = y3 – 2
Switch x and y in the equation.
x + 2 = y3
Add 2 to both sides of the equation.
Take the cube root of both sides. 3
x + 2 = y 3
x + 2 = y
Simplify.

16. Check It Out! Example 2 Continued
Because the inverse is a function,
The domain of the inverse is the range of f(x): R.
The range is the domain of f(x): R.
Check Graph both relations to see that they are symmetric about y = x.

17. You have seen that the inverses of functions are not necessarily functions. When both a relation and its inverses are functions, the relation is called a one-to-one function. In a one-to-one function, each y-value is paired with exactly one x-value.
You can use composition of functions to verify that two functions are inverses. Because inverse functions “undo” each other, when you compose two inverses the result is the input value x.

19. Example 3: Determining Whether Functions Are Inverses
Determine by composition whether each pair of functions are inverses. 1 3
f(x) = 3x – 1 and g(x) = x + 1
Find the composition f(g(x)).
Substitute x + 1 for x in f. 1 3
f(g(x)) = 3( x + 1) – 1 1 3
Use the Distributive Property.
= (x + 3) – 1
= x + 2
Simplify.

20. Example 3 Continued
Because f(g(x)) ≠ x, f and g are not inverses. There is no need to check g(f(x)).
Check The graphs are not symmetric about the line y = x.

21. Example 3B: Determining Whether Functions Are Inverses
For x ≠ 1 or 0, f(x) = and g(x) = 1 x
x – 1
Find the compositions f(g(x)) and g(f (x)).
= (x – 1) + 1
= x
= x
Because f(g(x)) = g(f (x)) = x for all x but 0 and 1, f and g are inverses.

22. Example 3B Continued
Check The graphs are symmetric about the line y = x for all x but 0 and 1.

23. Determine by composition whether each pair of functions are inverses.
Check It Out! Example 3a
Determine by composition whether each pair of functions are inverses. 3 2
f(x) = x + 6 and g(x) = x – 9
Find the composition f(g(x)) and g(f(x)).
f(g(x)) = ( x – 9) + 6 3 2
g(f(x)) = ( x + 6) – 9 2 3
= x –
= x + 9 – 9
= x
= x
Because f(g(x)) = g(f(x)) = x, they are inverses.

24. Check It Out! Example 3a Continued
Check The graphs are symmetric about the line y = x for all x.

25. 10 x - Check It Out! Example 3b f(x) = x2 + 5 and for x ≥ 0
Find the compositions f(g(x)) and g(f(x)).
f(g(x)) =
Substitute for x in f.
= x 10 x -
Simplify.
= x – 10 x + 30

26. Check It Out! Example 3b Continued
Because f(g(x)) ≠ x, f and g are not inverses. There is no need to check g(f(x)).
Check The graphs are not symmetric about the line y = x.

27. Lesson Quiz: Part I
1. Use the horizontal-line test to determine whether the inverse of each relation is a function.
A: yes; B: no

28. Lesson Quiz: Part II
2. Find the inverse f(x) = x2 – 4. Determine whether it is a function, and state its domain and range.
not a function
D: {x|x ≥ 4}; R: {all Real Numbers}

29. Lesson Quiz: Part III
3. Determine by composition whether f(x) = 3(x – 1)2 and g(x) = are inverses for x ≥ 0.
yes