1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Integration by Parts

  1. Oct 23, 2006 #1
    Hi all,

    I am having problems with the integral:

    [tex]\int e^{(ax)} cos(bx) dx [/tex]

    I have got to [tex] \frac{e^{ax} sin(bx)}{b} - \int \frac{a e^{ax} sin(bx)}{b} dx[/tex]

    After this I can only see myself going around in circles.

    Any help would be appreciated. :smile:


    The Bob (2004 ©)
  2. jcsd
  3. Oct 23, 2006 #2
    The integrals should be:

    [tex]\int e^{(ax)} cos(bx) dx [/tex]


    [tex] \frac{e^{ax} sin(bx)}{b} - \int \frac{a e^{ax} sin(bx)}{b} dx[/tex]

    LaTex doesn't seem to be editable anymore. :frown:

    The Bob (2004 ©)
  4. Oct 23, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Go again, taking the integral on the RHS Since you get a -cos(x), where you would normally have subtraction from your integration by parts you get addition, except the whole thing is negative anyway, so your new integral (which is a bunch of constants times eaxcos(bx) ) turns out negative. Add that to both sides, and multiply/divide by constants to isolate your original integral
  5. Oct 26, 2006 #4
    Cheers Office_Shredder. Sorry for the late reply, I have been very busy recently.

    Thanks so much again :biggrin:,

    The Bob (2004 ©)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook