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Homework Help: Need help on an Integral

  1. Feb 23, 2010 #1
    1. The problem statement, all variables and given/known data
    I am having trouble finding the right solution to this integral it is of the form int(1/x)dx=lnx+c but apparently im doing it wrong because of the other factors in the denominator
    integral(dx/[(k1+k2)x-k2*y]

    2. Relevant equations



    3. The attempt at a solution
    i get (1/(k1+k2))(ln(x/xi))-(x-xi)/k2*y but apparently the answer is
    (1/(k2+k2))ln[((k1+k2)x-k2*xi)/((k1+k2)xi-k2*xi)]
    so what am i doing wrong
     
  2. jcsd
  3. Feb 23, 2010 #2
    Re: Integral

    please note that i evalued the integrals for both from xi to x also
     
  4. Feb 23, 2010 #3

    Mark44

    Staff: Mentor

    Re: Integral

    Is this your integral?
    [tex]\int_{x_i}^x \frac{dt}{(k_1 + k_2)t - k_2 y}[/tex]

    It's unusual to have a variable as a limit of integration that is also the dummy variable of the integrand, so I switched the dummy variable to t.

    To do this integral, use u = (k_1 + k_2)t - k_2*y. Then du = (k_1 + k_2)dt. Can you finish this?
     
  5. Feb 23, 2010 #4
    Re: Integral

    Thank you mark I understand now, you are a great help!!!
     
  6. Feb 23, 2010 #5

    Mark44

    Staff: Mentor

    Re: Integral

    Aw, shucks!
     
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