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Regarding partial fractions

  1. Nov 15, 2003 #1
    hi, the problem is:


    ∫ (8x-17)/x^2+x-12 dx = ∫ .../(x+4)(x-3) dx

    so 8x - 17 = A(x-3) + B(x+4)

    (A+B)x + 4B - 3A

    so we have 2 eq and 2 unknown

    A+B = 8
    4B-3A = -17

    ... but the book says it's suppose to be 4A - 3B,.. I don't know what I did wrong.

    Please help.
     
  2. jcsd
  3. Nov 15, 2003 #2

    jamesrc

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    They probably just defined A and B differently than you did. (They must have said (8x-17)/(x2+x-12) = A/(x-3) + B/(x+4) ). You'll still get the right answer your way.
     
  4. Nov 15, 2003 #3
    the ending result is different...

    they got B = 7 and A = 1...


    I got B = 41/7, and A = 15/7
     
  5. Nov 15, 2003 #4

    Hurkyl

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    Well, the B and A you listed aren't solutions to the equations you got...
     
  6. Nov 15, 2003 #5

    Tom Mattson

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    You don't have to solve these like that.

    Starting from here, just plug in x=+3 and x=-4.

    x=3:
    8(3)-17=A(3-3)=B(3+4)
    7=7B
    B=1

    x=-4:
    8(-4)-17=A(-4-3)+B(-4+4)
    -49=-7A
    A=7
     
  7. Nov 15, 2003 #6
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