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Homework Help: Gaussian Integral Question

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data

    The integral of (x^n)(e^(-a*x^2)) is easier to evaluate when n is odd.
    a) Evaluate ∫(x*e^(-a*x^2)*dx) (No computation allowed!)
    b) Evaluate the indefinite integral of x*e^(-a*x^2), using a simple substitution.
    c) Evaluate ∫(x*e^(-a*x^2)*dx) [from o to +inf]
    d) Differentiate the previous result to evaluate ∫((x^3)(e^(-a*x^2))dx)

    2. Relevant equations

    ∫(e^(-a*x^2)*dx) = (1/2)√(∏/a)

    3. The attempt at a solution

    I thought that the easiest solution might be using integration by parts, but I ran into the issue of the range being different on the integrals, and I have no idea how else I can do this unless I can assume that the function is even...
    Last edited: Sep 3, 2013
  2. jcsd
  3. Sep 3, 2013 #2


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    What are the limits on the integrals?
  4. Sep 3, 2013 #3
    The first one is -inf to inf, second is indefinite, third is 0 to inf, and fourth is 0 to inf.
  5. Sep 3, 2013 #4


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    You don't need integration by parts for any of those. Take them one at a time. Start with a). The integrand isn't even, it's odd.
  6. Sep 3, 2013 #5
    Alright, I figured out the first one in class by drawing the graph for x and e^(-ax^2) and realized that the x made the function odd, as you said, and that the integral was then 0 for -inf to inf.

    I'm gonna try and tackle c) and d) because I have no idea where to start on b) with that substitution. My brain is stuck on integration by parts >_> if you could give me a hint or something as to where to begin with the indefinite integral, that would be great!
  7. Sep 3, 2013 #6
    Update: Just tried substituting u = a*x^2 into the integral, and it evaluated to -(1/2a)*e^(-a*x^2)

    I think it's right... but I'm not sure, the substitution worked though because the du = 2ax dx which means we can just throw the constants in there.
  8. Sep 3, 2013 #7
    Another Update: The above equation is correct because it yields the correct value for part c).

    Now I'm just stuck on d) where it's asking me to differentiate... More later if I figure it out.
  9. Sep 3, 2013 #8


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    Try differentiating it and see if you recover the integrand.
  10. Sep 3, 2013 #9
    Got it! Taking the derivative with respect to a of part c) yields the answer! :D
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