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Homework Help: Online problem

  1. Mar 25, 2006 #1
    I have to solve this online problem for homework and was wondering if anyone could help me with it, i can find the answer by graphing and solving, but don't know how to explain the mistake....

    http://www.dougshaw.com/findtheerror/FTEusub.html [Broken]

    thanks for the help
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Mar 25, 2006 #2


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    Homework Helper

    If you have an integral from a to b (in this case, -1 to 1), u has to be a 1-1 continuously differentiable function of x with non-zero derivative. That is, u has to be a function of x defined on all of [-1, 1]. It has to be 1-1, so u has to take on a different value for every x. u has to be continuous, differentiable (with respect to x, i.e. you can find du/dx), and du/dx has to be continuous. Moreover, du/dx can never be 0, otherwise dx = du/u', so and if u' is ever 0, then when you change your variables, the integrand will have 0 in the denominator at some point, which isn't allowed. Your u substitution fails about half of these criteria.

    For one, it is not defined when x = 0. Second, 1/x4 = 1/(-x)4, so u repeats the value 1/24 at both x = 1/2 and x = -1/2.
  4. Mar 25, 2006 #3
    thank you

    thank you!
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