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: Partial Fractions problem not matching Wolfram Alpha

  1. May 19, 2012 #1
    1. The problem statement, all variables and given/known data

    Solve by partial fractions.

    3. The attempt at a solution

    ∫A/(x-1) +B/(x-1)2 + C(x+3)

    after setting up the partial fractions and multiplying each term by LCD:

    10x-2x2= A(x-1)(x+3) + B(x+3) + C(x-1)2

    10x-2x2= A(x2+2x-3) +Bx+3B +Cx-C

    10x-2x2= Ax2+2Ax-3A+Bx+3B+Cx-C

    10x-2x2= x2(A+C) +x(2A+B-2C) + (3B+C-3A)

    System of Equations:

    A+C = -2
    3B+C-3A =0

    Solving these, I get A=23. B=14. C=25.

    Putting them back in their partial fractions form and integrating I get:

    23*lnlx-1l - 14/(x-1) + 25*lnlx+3l

    However Wolfram gives me essentially the same terms but different coefficients: http://www.wolframalpha.com/input/?i=integral+of+%2810x-2x%5E2%29%2F%28%28x-1%29%5E2*%28x%2B3%29%29

    What am I doing wrong???
  2. jcsd
  3. May 19, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You're not solving for A, B, and C correctly. Your equations are fine, but your numerical answers are not. You can easily see this by plugging the results you got for A and C into the first equation.
  4. May 19, 2012 #3
    THANK YOU! Got the right answer now. Algebra...a slippery fooler.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook