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Homework Help: First Order Homogeneous Equation

  1. Sep 19, 2009 #1
    1. The problem statement, all variables and given/known data

    (4y4-9x2y2-144)dx - (5xy3)dy = 0


    2. Relevant equations
    substitute y = xv
    dy = dx v + dv x


    3. The attempt at a solution
    after substituting i got

    (4x4v4-9v2x4-14x4)dx - (5v3x4)dx.v + dv.x

    = (4v4-9v2-14)dx - 5v3(dx.v + dv.x) = 0
    = dx(4v4-9v2-14-5v4)+dv(-5v3x)= 0
    dx/x = (-5v3dv)/(v4-9v2-14)

    dx/x = ((2v)/(v2+2) - (7v)/(v2+7)) dv

    derive both sides

    ln(x) = ln(v2+2) - 3.5ln(v2+7)
    x + c = (v2+2) / (v2+7)3.5

    c = (((y/x)2+2) / ((y/x)2+7)3.5) - x


    what am i doing wrong?
     
  2. jcsd
  3. Sep 19, 2009 #2

    HallsofIvy

    User Avatar
    Science Advisor

    What reason do you have for thinking you are doing anything wrong?
     
  4. Sep 19, 2009 #3
    webworks (the online assignment thing my school uses) is telling me my answer is incorrect

    nvm i know where the mistake was made
     
    Last edited: Sep 19, 2009
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