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 partst

  1. Feb 27, 2010 #1
    1. The problem statement, all variables and given/known data

    The the problem is integrate ln(sqrt(x^2-4)

    2. Relevant equations

    For this one u-sub does not work, which leads me to think that integration by parts is the way to go but am not sure how to finish it.

    3. The attempt at a solution

    u =1/2ln(x^2-4)
    du = (1/2)(2x/(x^2-4)) = x^2/(x^2-4)

    dv = dx ----> int dx = x = v

    x(1/2ln(x^2-4)) - int (x^2)/(X^2-4)

    I dont know what to do about the part in bold. Im thinking integration by part again but that doesnt look like it will work, unless there is one more integration to do. Any idea what i can try?

  2. jcsd
  3. Feb 27, 2010 #2
    I think parts can work again, since 1 / x^2 - 4 is easy enough to integrate, then you'll need to integrate xarctan(x/2) which should be fine
  4. Feb 27, 2010 #3


    User Avatar
    Science Advisor
    Homework Helper

    Use polynomial division to write x^2/(x^2-4)=1+4/(x^2-4). Looks more to me like 'partial fractions' than 'parts'.
  5. Feb 27, 2010 #4
    Write it as
    ln(x^2 - 4)^1/2

    and also note that it's a difference of 2 squares. Then just use the log properties to reduce this to something easier and you can then use integration by parts.
  6. Feb 27, 2010 #5
    Thanks Dave! I did not see that, but it certainlly makes sense now.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook