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Homework Help: Integration by parts problem.

  1. Sep 8, 2009 #1
    1. The problem statement, all variables and given/known data
    Let F(b) be the exact area under the graph of y = x2*e-x between x=0 and x=b for b>0. Find the formula for F(b).

    2. Relevant equations
    int(uv')= uv - int(vdu)

    3. The attempt at a solution
    u = x2 and dv = e-x, thus u'=2xdx and v=-e-x.

    y= -x2*e-x - -2*integral(xe-x).
    = -x2*e-x -2xe-x-2e-x.

    This does not yield the correct equation, so where did i make a mistake?
  2. jcsd
  3. Sep 8, 2009 #2


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    Did you remember that this is a definite integral from 0 to b?
  4. Sep 8, 2009 #3
    yes, and this resulted in the previous answer with b's subbed in for the x's then +2 since the only term left would be - -2 when zero is plugged in.
  5. Sep 8, 2009 #4


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    Gold Member

    Then I don't see what's wrong with your answer. Why do you think it's wrong?
  6. Sep 8, 2009 #5
    it is a webassign problem, so it gives me a red x when I am wrong. I probably just need to fix notation somewhere, thank you for verifying that the integral is correct though :).
  7. Sep 8, 2009 #6
    Issue resolved, i accidentally had a double - that i had to check over a couple times to notice. Man I hate webassign lol.
  8. Sep 8, 2009 #7
    haha SBU im assuming?

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