Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Integration by parts is wrong
  1. Integration by parts (Replies: 1)

  2. Integration by parts (Replies: 7)

  3. Integration By Parts (Replies: 2)

Loading...