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Homework Help: Evaluate Integral

  1. Sep 7, 2008 #1
    ∫x^4 e^-x dx

    ok this equation looks like this integral
    ∫u^n e^au du= (1/a) u^n e^au - n/a ∫u^n-1 e^au du

    i did integration by parts and i ran in circles, and i tried substituting u=-x

    but i couldn't do much with it, so what can i do
  2. jcsd
  3. Sep 7, 2008 #2
    [tex]\int x^4 e^{-x} dx = -x^4 e^{-x} + \int 4 x^3 e^{-x} dx [/tex]
    Itterate this for the integral on the right untill the x term disapears.
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