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Definite Integrals with inverse of function

  • Thread starter eprparadox
  • Start date
  • #1
138
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Homework Statement


Suppose f(x) is continuous and decreasing on the closed interval 5 <= x < 13, that f(5) = 9, f(13) = 5 and that the

integral of f(x) from 5 to 13 is 70.64758.

Then the integral of f^-1(x) from 5 to 9 is equal to what?

(Note: f^-1(x) is the inverse of f(x))


Homework Equations






The Attempt at a Solution



I really don't know how to solve this problem. I know that f^-1(f(x)) = x. Any ideas would be great. Thanks.
 

Answers and Replies

  • #2
392
0
You want to find [tex]\int_{u=5}^9 f^{-1}(u)\,du[/tex]


Try the substitution u=f(x)
 
  • #3
726
1
f is a positive monotone function on [a,b] where 0 < a < b and f has an inverse [tex] f^-1 [/tex]. Set [tex] \alpha = f(a), \beta = f(b) [/tex] and then use this formula:[tex] \int_{\alpha}^{\beta} f^-1(y) dy = b \beta - a \alpha - \int_a^b f(x) dx [/tex].
 
  • #4
f is a positive monotone function on [a,b] where 0 < a < b and f has an inverse [tex] f^-1 [/tex]. Set [tex] \alpha = f(a), \beta = f(b) [/tex] and then use this formula:[tex] \int_{\alpha}^{\beta} f^-1(y) dy = b \beta - a \alpha - \int_a^b f(x) dx [/tex].
This formula can be seen by drawing an example graph. Along the x-axis, you are integrating f(x)dx. ALong the y-axis you are integrating f-1(y)dy. Draw some rectangles and add/subtract areas to get the formula.
 

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