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Math Help - Integration by parts? help plz

  1. #1
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    Integration by parts? help plz

    integral 0 to infinite (x^ 3)(e^-8x) cos(5x) dx

    should I do integrate by part?
    help me
    Last edited by CaptainBlack; October 22nd 2010 at 11:36 PM.
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  2. #2
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    Please use brackets.

    Is it: \displaystyle \int_{0}^{\infty} x^3 \cdot e^{-8x} \cdot \cos (5x) dx

    Or is it: \displaystyle \int_{0}^{\infty} x^{3e^{-8x}} \cdot \cos (5x) dx
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  3. #3
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    1st one. ! thanks. how did you do it?
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  4. #4
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    To type equations like I did, you use LaTeX to do it. LaTeX Tutorial.

    Yes, you would use integration by parts. For the \cos (5x) part you would also need to use integration by u-substitution as well as integration by parts.

    \displaystyle \int f'(x)g(x)h(x)dx = f(x)g(x)h(x) - \int f(x)g'(x)h(x)dx - \int f(x)g(x)h'(x)dx
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  5. #5
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    Quote Originally Posted by ddcamp View Post
    integral 0 to infinite (x^ 3)(e^-8x) cos(5x) dx

    should I do integrate by part?
    do you have to find the actual value of the improper definite integral, or do you just have to show that it converges?
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  6. #6
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    I have to find the actual value :S :S thanks!
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