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Integration by parts is wrong

  1. Sep 27, 2014 #1
    ∫(x2 + 7x) cosx dx

    If I make v = (x2 + 7x) and du = cosx dx I get
    ((x2 + 7x) sinx)/2

    If I make v = cosx and du = (x2 + 7x) dx I get
    ((x3/3 + 7x2/2) cosx)/2

    using the form X=Y-X to X=Y/2

    Neither are correct, what did I do wrong?
  2. jcsd
  3. Sep 27, 2014 #2
    Integration by parts:
    ∫u dv = uv - ∫v du

    You are doing the integration by parts incorrectly.
    Last edited: Sep 27, 2014
  4. Sep 27, 2014 #3


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    I recommend you expand the integrand (x2 + 7x) cosx into x2 cos x + 7x cos x, and integrate the two resulting terms.
    I would start with integrating the second term of this expansion first.
  5. Sep 27, 2014 #4
    K thanks
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