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A non-exact nonlinear first ODE to solve

  1. Jun 14, 2016 #1
    1. The problem statement, all variables and given/known data

    Solve the following equation.

    2. Relevant equations

    ( 3x2y4 + 2xy ) dx + ( 2x3y3 - x2 ) dy = 0

    3. The attempt at a solution

    M = ( 3xy4 + 2xy )
    N = ( 2x3y3 - x2 )

    ∂M/∂y = 12x2y3 + 2x
    ∂N/∂x = 6x2y3 - 2x

    Then this equation looks like that the integrating factor is (xM-yN).
    IF = x3y4 + 3x2y

    then the new equation would be:

    (3xy^3+2)/(x^2y^3+3x)dx + (2xy^3-1)/(xy^4+3y)dy = 0

    but then if we find that this new equation is exact or not, it proves that this is not exact.
    If so what should I do?
    Please help...

    The given answer is:
    x3y2 + x2/y = c
  2. jcsd
  3. Jun 14, 2016 #2


    Staff: Mentor

    The above should be ##M = 3x^2y^4 + 2xy##
    You have a mistake above.
    If ##M = 3x^2y^4 + 2xy## then ##M_y = 12x^2y^3 + 2x##
    Since My ≠ Nx, that's enough to show that the equation is not exact.

    I taught diff. equations a number of times, but this isn't a trick that I remember. Where does xM - yN come from?
    I'm not following this at all. If you can find an integrating factor, you multiply both sides of the original diff. equation by it. If it really is an integrating factor, then the new equation will be exact, and you can get the solution.
  4. Jun 14, 2016 #3


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  5. Jun 14, 2016 #4


    Staff: Mentor

    I understand that he/she was trying to come up with an integrating factor. The link below presents the usual technique to start with when you're faced with an inexact equation: seeing if a function of x (##\mu(x)##) in the PDF below, and proceeding to a function of y alone (##\nu(y)##) if the previous try didn't work. What threw me was the immediate jump to xM - yN, which as you point out, is not the integrating factor.
  6. Jun 16, 2016 #5
    There was a misinterpretation in the question by me... I solved it by myself. Thanks for the concern.
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