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Homework Help: Help with first integral of PDE

  1. Mar 2, 2010 #1
    Hey guys, I'm having a little difficulty with a pde I'm trying to solve. It boils down to solving for a first integral. I don't want the answer, but I'd be glad to get a little help. We have the system:

    [tex]\frac{dx}{x^2} = \frac{dy}{y^2} = \frac{dz}{xy(z^2 + 1)}[/tex]

    We can use the first two and find that

    [tex] \frac{1}{x} - \frac{1}{y} = c [/tex]

    I need to use all three to find a second function which is constant here. I've tried using the compendo and dividendo rule and I can't seem to get anywhere... I'm hoping just for a slight hint because I want to solve it myself but I'm really stuck at this point.

  2. jcsd
  3. Mar 2, 2010 #2


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    Homework Helper

    Here are two hints you might find helpful:

    (1) You have a relationship between x and y, so you can isolate for x or y, then substitute the result into the original system of equations.

    (2) You can manipulate the original system as if the differentials (dx, dy, dz) were parts of a fraction. You can multiply both sides by the same factor, for example, or cancel out common factors, just like with fractions.
  4. Mar 2, 2010 #3
    right... the problem is this solution isn't as easy as all that... there is a more trivial example solved with

    [tex] \frac{dx}{x^2} = \frac{dy}{y^2} = \frac{dz}{z(x+y)} [/tex]

    Which can be solved by doing some proper addition and subtraction: so I know the idea. Its a matter of getting a form [tex]\frac{g*dx + h*dy}{f(x,y)} [/tex] such that that solution is the differential of a higher form... as the dz part is quite easy, we can integrate it to arctan, we just need a better function of the xs I think...
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