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Homework Help: Working the differential equation dy/dx = x-y/x+y

  1. Mar 4, 2009 #1
    1. The problem statement, all variables and given/known data

    Solve [tex]\frac{dy}{dx} = \frac{x-y}{x+y}[/tex]

    2. Relevant equations

    Homogeneous differential equation rules = [tex] v = \frac{y}{x} [/tex][tex] \frac{1}{y} = \frac{x}{y}[/tex] [tex] \frac{dy}{dx} = v + x\frac{dv}{dx}[/tex]

    3. The attempt at a solution

    [tex]\frac{dy}{dx} = \frac{x}{x+y}-\frac{y}{x+y} = \frac{1}{1+\frac{y}{x}} - \frac{1}{1+\frac{x}{y}}[/tex]

    [tex]x+\frac{dv}{dx} = (1+v)^-1-(1+1/v)^-1 [/tex]

    I'd like to know if what I've done here looks good so far? I'm not getting the right answer when I complete the integration, so I'm curious to know if I'm making an error after this point or if I've just completely set the problem up wrong. Thanks for any help!
  2. jcsd
  3. Mar 4, 2009 #2


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    Gold Member

    Check your last step its supposed to be [tex]v+x\frac{dv}{dx}[/tex]
  4. Mar 5, 2009 #3


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    Science Advisor

    As djeitnstine pointed out, your left side should be v+ x dv/dx. The right side is
    [tex]\frac{1}{1+ v}- \frac{1}{1+ \frac{1}{v}}[/tex]
    Multiplying the numerator and denominator of the last fraction by v, this is
    [tex]\frac{1}{1+ v}- \frac{v}{v+ 1}[/tex]
    which is equal to ?
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