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Draw graphs of the functions f  and  f1 from [link] .

Graph of f(x) and f^(-1)(x).
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Is there any function that is equal to its own inverse?

Yes. If f=f1, then f(f(x))=x, and we can think of several functions that have this property. The identity function does, and so does the reciprocal function, because

11x=x

Any function f(x)=cx, where c is a constant, is also equal to its own inverse.

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Key concepts

  • If g(x) is the inverse of f(x), then g(f(x))=f(g(x))=x. See [link] , [link] , and [link] .
  • Only some of the toolkit functions have an inverse. See [link] .
  • For a function to have an inverse, it must be one-to-one (pass the horizontal line test).
  • A function that is not one-to-one over its entire domain may be one-to-one on part of its domain.
  • For a tabular function, exchange the input and output rows to obtain the inverse. See [link] .
  • The inverse of a function can be determined at specific points on its graph. See [link] .
  • To find the inverse of a formula, solve the equation y=f(x) for x as a function of y. Then exchange the labels x and y. See [link] , [link] , and [link] .
  • The graph of an inverse function is the reflection of the graph of the original function across the line y=x. See [link] .

Section exercises

Verbal

Describe why the horizontal line test is an effective way to determine whether a function is one-to-one?

Each output of a function must have exactly one output for the function to be one-to-one. If any horizontal line crosses the graph of a function more than once, that means that y -values repeat and the function is not one-to-one. If no horizontal line crosses the graph of the function more than once, then no y -values repeat and the function is one-to-one.

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Why do we restrict the domain of the function f(x)=x2 to find the function’s inverse?

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Can a function be its own inverse? Explain.

Yes. For example, f(x)=1x is its own inverse.

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Are one-to-one functions either always increasing or always decreasing? Why or why not?

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How do you find the inverse of a function algebraically?

Given a function y=f(x), solve for x in terms of y. Interchange the x and y. Solve the new equation for y. The expression for y is the inverse, y=f1(x).

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Algebraic

Show that the function f(x)=ax is its own inverse for all real numbers a.

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For the following exercises, find f1(x) for each function.

f(x)=x+3

f1(x)=x3

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f(x)=x+5

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f(x)=2x

f1(x)=2x

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f(x)=3x

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f(x)=xx+2

f1(x)=2xx1

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f(x)=2x+35x+4

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For the following exercises, find a domain on which each function f is one-to-one and non-decreasing. Write the domain in interval notation. Then find the inverse of f restricted to that domain.

f(x)=(x+7)2

domain of f(x):[7,);f1(x)=x7

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f(x)=(x6)2

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f(x)=x25

domain of f(x):[0,);f1(x)=x+5

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Given f(x)=x2+x and g(x)=2x1x:

  1. Find f(g(x)) and g(f(x)).
  2. What does the answer tell us about the relationship between f(x) and g(x)?

a.  f(g(x))=x and g(f(x))=x. b. This tells us that f and g are inverse functions

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Practice Key Terms 1

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Source:  OpenStax, Algebra and trigonometry. OpenStax CNX. Nov 14, 2016 Download for free at https://legacy.cnx.org/content/col11758/1.6
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