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Math Help - Is this differential equation separable?

  1. #1
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    Is this differential equation separable?

    Is this separable? I'm trying, but I'm not sure if I'm on the right track:

    y' = (x+y)/(x+2y)

    dy/dx = (x+y)/(x+2y)

    multiply both sides by dx:

    dy = (x+y)/(x+2y) dx

    Now I am not sure what to do because if I get (x+2y) to the other side, it'd still have an x and would be attached to the dy as well.
    Last edited by NeedsHelpPlease; August 9th 2012 at 01:56 PM.
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  2. #2
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    Re: Is this differential equation separable?

    Quote Originally Posted by NeedsHelpPlease View Post
    Is this separable? I'm trying, but I'm not sure if I'm on the right track:

    y' = (x+y)/(x+2y)

    dy/dx = (x+y)/(x+2y)

    multiply both sides by dx:

    dy = (x+y)/(x+2y) dx

    Now I am not sure what to do because if I get (x+2y) to the other side, it'd still have an x and would be attached to the dy as well.
    I'd do it like this... Make the substitution \displaystyle \begin{align*} v = \frac{y}{x} \implies y = v\,x \implies \frac{dy}{dx} = v + x\,\frac{dv}{dx} \end{align*}, then

    \displaystyle \begin{align*} \frac{dy}{dx} &= \frac{x + y}{x + 2y} \\ \frac{dy}{dx} &= \frac{1 + \frac{y}{x}}{1 + 2\left(\frac{y}{x}\right)} \\ v + x\,\frac{dv}{dx} &= \frac{1 + v}{1 + 2v} \\ x\,\frac{dv}{dx} &= \frac{1 + v}{1 + 2v} - v \\ x\,\frac{dv}{dx} &= \frac{1 - 2v^2}{1 + 2v} \\ \frac{1 + 2v}{1 - 2v^2}\,\frac{dv}{dx} &= \frac{1}{x} \end{align*}

    Go from here.
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