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Math Help - Integrating by parts

  1. #1
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    Integrating by parts

    Simple question, but marked me wrong. Am I forgetting something?

    To use the integration-by-parts formula
    integral udv= uv - integral vdu


    we must choose one part of
    8x cos 5x dx

    to be u, with the rest becoming dv.

    Since the goal is to produce a simpler integral, we will choose u = 8x.
    This means that dv = cos5xdx <----says this is wrong)
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  2. #2
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    Re: Integrating by parts

    Uhm,

    \int 8x \cos {5x}\, dx = \int 8x \left ( \frac{1}{5}\sin {5x} \right )'\, dx =8x\cdot \frac{1}{5}\sin {5x} -\int (8x)'\cdot \frac{1}{5}\sin {5x}\, dx
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  3. #3
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    Re: Integrating by parts

    Veileen that's not very helpful.

    You made the correct substitution, nothing is wrong with that.
    Last edited by Shakarri; March 8th 2013 at 11:39 AM.
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  4. #4
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    Re: Integrating by parts

    so dv= 1/5 sin 5x ?


    PS: wait...that would be the antiderivative of dv...dv would be cos5xdx but Im not sure why I'm marked wrong


    Never mind, I needed the parenthesis.....stupid online hw.
    Last edited by Steelers72; March 8th 2013 at 02:05 PM.
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