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Help with an easy ODE

  1. Jan 14, 2010 #1
    so i'm in my second week of my first differential equations class, and i can't seem to get this problem.

    Show that 5x2y2 - 2x3y2 = 1 is an implicit solution of the differential equation x(dy/dx) + y = x3y3 on the interval 0 < x < 5/2

    for dy/dx, i got (3xy-5y)/(5x-4x2), i'm pretty sure it's right, but if it's not then it might be why i can't quite get this. i also solved for y explicitly, but then i realized i could just factor it out.

    well that's what i did, and all i end up with is just a big polynomial... i'm sure i'm doing it wrong. help please?

    edit: alright alright i just saw the sticky... truth be told i just google differential equations forum and signed up real quick to post this, so i was in a hurry. sorry i posted this is the wrong forum :(
     
    Last edited: Jan 14, 2010
  2. jcsd
  3. Jan 15, 2010 #2

    HallsofIvy

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    No, that derivative is not right. Just use "implicit differentiation" to get [itex]10xy^2dx+ 10x^2ydy- 5x^2y^2dx- 4x^3y dy= 0[/itex]. Rearrange those to [itex](10xy^2- 5x^2y)dx+ (10x^2y- 4x^3y)dy= 0[/itex]. That, now, is the same as [itex]dy/dx= -(10xy^2- 5x^2y)/(10x^2y- 4x^3y)[/itex]. Factor and reduce that.

     
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