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Prove that the function in defined

  1. Aug 24, 2007 #1


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    1. The problem statement, all variables and given/known data

    f(x) is defined as f(x) = 1/((ln(x+1))^2 + 1) for all x>-1 and f(x)=0 for x=-1.

    1)Prove that the function [tex]F(x) = \int^{x^2 + 2x}_{0} f(t)dt[/tex]
    is defined and has a derivative in R.
    2)g(x) is defined as g(x)=f(x) for x>-1 and g(x)=-1 for x=-1.
    Also, [tex]G(x) = \int^{x^2 + 2x}_{0} g(t)dt[/tex]
    Is G(x) defined in R? Does it have a derivative?

    2. Relevant equations

    3. The attempt at a solution

    1) By taking the limit of f(x) at x=0 we see that f is continues for all x>=1 and since
    x^2 + 2x >= -1 for all x in R F(x) is defined and is has a derivative from the chain rule.

    2)Since f(x)=g(x) for all x=/=-1 F(x)=G(x) and so the answer to both questions is yes.

    Are those right? I think that the answer to (2) is wrong but why?
    Last edited: Aug 24, 2007
  2. jcsd
  3. Aug 24, 2007 #2


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    Seems fine to me. For 2), why would you think changing the value of a function at a single point could change the integral? The set {-1} has measure 0.
  4. Aug 24, 2007 #3


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    Thanks a lot.
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