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Homework Help: Integral question

  1. Aug 11, 2007 #1


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    Gold Member

    1. The problem statement, all variables and given/known data
    f is a function defined as: for all x>-1 f(x) = 1/((ln(x+1))^2 + 1) and for x=-1 f(x)=0.
    a) Prove that [tex]F(x) = \int^{x^2 + 2x}_{0} f(t)dt[/tex] is defined in R and has a derivative there. Find the derivative.

    b) g is defined as: for all x>-1 g(x) = f(x) and for x=-1 g(x) = -1.
    Is [tex]G(x) = \int^{x^2 + 2x}_{0} g(t)dt[/tex] defined in R? Does it have a derivative there?

    2. Relevant equations

    3. The attempt at a solution
    a)first I proved that f is continues for all x>=-1 Then since x^2+2x>0 for all x in R F(x) is defined and it's derivative is easy to get with the chain rule. I got:
    F'(x) = (2x+2)/((ln(x^2 + 2x +1))^2 + 1)

    b)Since g differs from f in only one point F(x) = G(x) and so G is defined and has a derivative for all x in R.

    Is that right? I'm mostly worried about (b).
  2. jcsd
  3. Aug 12, 2007 #2

    Gib Z

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    Looks good to me :) Other than the small error than G is defined and has a derivative for all x > -1, not all real values.
  4. Aug 12, 2007 #3


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    In (2), it helps to notice, as you said for (1), that x2+ 2x is never negative: the interval of integration, 0 to x2+ 2x, does not contain -1 and so what happens there is irrelevant.
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