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Homework Help: Second order differential equation

  1. Feb 10, 2010 #1
    1. The problem statement, all variables and given/known data

    What is the general solution to y'' - 2(y')^2 = 0 ?

    2. Relevant equations

    3. The attempt at a solution

    Let u = y '

    u ' - 2u^2 = 0
    u ' = 2u^2
    u = (2 / 3)u^3 + C

    This cannot be solved using separation of variables, what is done?
  2. jcsd
  3. Feb 11, 2010 #2
    your first step is correct when you chose u=y' , then you got the equation:
    u'-2u^2=0 ,then : u'=2u^2

    I believe that the last step you did is not correct, as far as i remember you should write u' = du/dx (or du/dt) according to what function is y originally, then from there you can use separation of variables .. :)
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