1.5 INVERSE FUNCTIONS Copyright © Cengage Learning. All rights reserved.

2 Find inverse functions informally and verify that two functions are inverse functions of each other. Use graphs of functions to determine whether functions have inverse functions. What You Should Learn

3 Inverse Functions

4 Recall that a function can be represented by a set of ordered pairs. For instance, the function f (x) = x + 4 from the set A = {1, 2, 3, 4} to the set B = {5, 6, 7, 8} can be written as follows. f (x) = x + 4: {(1, 5), (2, 6), (3, 7), (4, 8)} In this case, by interchanging the first and second coordinates of each of these ordered pairs, you can form the inverse function of f, which is denoted by f –1.

5 Inverse Functions It is a function from the set B to the set A, and can be written as follows. f –1 (x) = x – 4: {(5, 1), (6, 2), (7, 3), (8, 4)} Note that the domain of f is equal to the range of f –1, and vice versa, as shown in Figure Also note that the functions f and f –1 have the effect of “undoing” each other. Figure 2.66

6 Inverse Functions In other words, when you form the composition of f with f –1 or the composition of f –1 with f, you obtain the identity function. f (f –1 (x)) = f (x – 4) = (x – 4) + 4 = x f –1 (f (x)) = f –1 (x + 4) = (x + 4) – 4 = x

7 Example 1 – Finding Inverse Functions Informally Find the inverse of f (x) = 4x. Then verify that both f (f –1 (x)) and f –1 (f (x)) are equal to the identity function. Solution: The function f multiplies each input by 4. To “undo” this function, you need to divide each input by 4. So, the inverse function of f (x) = 4x is

8 Example 1 – Solution You can verify that both f (f –1 (x)) = x and f –1 (f (x)) = x as follows. cont’d

9 Definition of Inverse Function

10 Definition of Inverse Function Do not be confused by the use of –1 to denote the inverse function f –1. In this text, whenever f –1 is written, it always refers to the inverse function of the function f and not to the reciprocal of f (x). If the function g is the inverse function of the function f, it must also be true that the function f is the inverse function of the function g. For this reason, you can say that the functions f and g are inverse functions of each other.

11 The Graph of an Inverse Function

12 The Graph of an Inverse Function The graphs of a function f and its inverse function f –1 are related to each other in the following way. If the point (a, b) lies on the graph of f, then the point (b, a) must lie on the graph of f –1, and vice versa. This means that the graph of f –1 is a reflection of the graph of f in the line y = x, as shown in Figure Figure 2.67

13 Example 2 – Finding Inverse Functions Graphically Sketch the graphs of the inverse functions f (x) = 2x – 3 and on the same rectangular coordinate system and show that the graphs are reflections of each other in the line y = x.

14 Example 2 – Solution The graphs of f and f –1 are shown in Figure It appears that the graphs are reflections of each other in the line y = x. You can further verify this reflective property by testing a few points on each graph. Figure 2.68

15 Example 2 – Solution Note in the following list that if the point (a, b) is on the graph of f, the point (b, a) is on the graph of f –1. Graph of f (x) = 2x – 3 Graph of (–1, –5) (–5, –1) (0, –3) (–3, 0) (1, –1) (–1, 1) (2, 1) (1, 2) (3, 3) (3, 3) cont’d