3.6 – Inverse Functions Exchanging input and output values of a function or relation is called an inverse relation. If the inverse is a function, then we have an inverse function. The key idea of inverses is that for a point x , y on a graph, the point y , x is on its inverse graph. EXAMPLE 1: Sketch the graph of the quadratic function f which passes through the following points: (6,5) (0,5)
(3, 4) Graph the inverse on the same set of axes and describe the relationship between the function and its inverse.
Let f be a function. If a, b is a point on f, then b, a is a point on the graph of its inverse. The graph of the inverse of f is a reflection of the graph of f across the line y x . To find an inverse from an equation, write the function f (x) in terms of x and y, switch the x and y, and then solve for y. Call the result g(x) . EXAMPLE 2: Find g(x) , the inverse of f (x) 2x 1 .
EXAMPLE 3: Find g(x) , the inverse of f (x) 4 x 2 2x .
To determine whether an inverse of a function f is also a function, every output of f must correspond to exactly one input, making f a one-to-one function. A function f is one-to-one if f (a) f (b) implies that a b . If a function is one-to-one, then its inverse is also a function. A function is one-to-one if it passes the horizontal line test. (NOTE: a relation is a function if it passes the vertical line test and if it passes the horizontal line test, its inverse is also a function!!)
EXAMPLE 4: Graph each function below and determine whether the function is one-to-one. If so, graph its inverse function: a) f (x) 3x 5 2x 4 3x 3 x 2
Note that f is always ____________________, so it passes the ___________________________and is _____________________; its inverse is a ________________ that is also always ______________________. b) g(x) 0.5x 3 x 1
Note that g is ____________________ AND ____________________, so it does NOT pass the ________________________ and it is NOT _____________________ and its inverse is NOT a function!! c) h(x) 2 0.8 x 3
Note that h is always ____________________, so it passes the ________________________ and is _____________________ and its inverse is also always ______________________. Let f be a function. TFAE (the following are equivalent): The inverse of f is a function f is one-to-one the graph of f passes the Horizontal Line Test The inverse function, if it exists, is written as f 1 where: if y f (x) , then x f 1 (y) the notation f 1 does not mean
When a function is not one-to-one, it is possible to create an inverse function by restricting the domain of the function to the part that that is one-to-one. EXAMPLE 5: Find an interval on which the function f (x) x 2 3 is one-to-one, and find f 1 on that interval.
The inverse of a function f sends each output of f back to the input it came from, that is: f a b exactly when f 1 b a (a is sent to b under f exactly when b gets sent back to a under f 1 )
A one-to-one function f and its inverse function f 1 have these properties:
f 1 f x x for every x in the domain of f
f 1 x f f
x for every x in the domain of f 1
Also, any two functions having both properties are one-to-one and are inverses of each other. IN OTHER WORDS: If you compose a function with its inverse, you will get x.
EXAMPLE 6: Assume f is a one-to-one function. a) If f 3 10, find f 1 10
b) If f 1 7 3, find f 3
2x 3x . Use composition to verify that f and g are inverses of each and g(x) 3 x x 2 other. (In other words, compute f g and g f and show that the result is x.)
EXAMPLE 7: Let f (x)