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Thread: [SOLVED] Help with "Exact Differential Equations"

  1. #1
    Sep 2009

    [SOLVED] Help with "Exact Differential Equations"

    I need to find integrating factor, i will solve the equation once the integrating factor is found.

    $\displaystyle (10x^2+2xy+6y^2)dx+(3x^2+4xy+5y^2)dy=0$

    Where the equation is in the form of:$\displaystyle (M)dx+(N)dy=0$

    $\displaystyle \dfrac{d}{dy}[M]=2x+12y$

    $\displaystyle \dfrac{d}{dx}[N]=6x+4y$

    $\displaystyle \dfrac{d}{dy}[M] \not = \dfrac{d}{dx}[N]$ Therefore Differential equation is NOT exact...yet, I still need to find the integrating factor.

    I need to find an "integrating factor" using this special case:
    $\displaystyle u(x,y)=ax+by$
    $\displaystyle \mu=\mu(ax+by)$ if an only if $\displaystyle \dfrac{M_{y}-N_{x}}{aN-bM}=R(u)=\dfrac{\mu'(u)}{\mu(u)}$

    $\displaystyle \dfrac{M_{y}-N_{x}}{aN-bM}$ should simpify down to 1 (i think, professor didnt say for this special case ) by making $\displaystyle a$ and $\displaystyle b$ appropriote constant values. Here is what I get:

    $\displaystyle \dfrac{-4x+8y}{(3a-10b)x^2+(4a-2b)xy+(5a-6b)y^2}$

    I cannot make $\displaystyle a$ and $\displaystyle b$ values that will simplify this fraction down to "1" unless they are 0, which does not work.

    I need to find the "integrating factor" but messed up or am doing it wrong, in either case any help would be appreciated. Thanks!
    Last edited by snaes; Feb 1st 2010 at 07:58 PM.
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  2. #2
    Super Member
    Aug 2008
    Hey, isn't it homogeneous? Why not solve it via $\displaystyle y=vx$?
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  3. #3
    Sep 2009
    That would make this problem a lot easier, except I dont know how to do a problem similar to this, where I need to find an integrating factor in the form $\displaystyle ax+by$, to make the equation exact.
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