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Homework Help: Variation of parameters

  1. Sep 13, 2007 #1
    1. The problem statement, all variables and given/known data

    x^2y''-2xy'+2y=x^3cosx

    Find a general solution by using variation of parameters

    2. The attempt at a solution

    I already solved this one, but I have 4 questions:

    1. I found the solutions x and x^2 to the homogeneous equation by inspection. Is this the only way to do it?

    2. How about the solutions -x and -x^2? I first tried the solutions x^2 and -x^2, and that gave me 0 as the particular solution...Why can't I choose these two?

    3. When calculating the Wronski, how do I know which solution is y1 and y2? I first chose y1 as x^2, and that gave me a different solution...

    4. Why are there no constants of integration?
     
  2. jcsd
  3. Sep 14, 2007 #2

    HallsofIvy

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    Science Advisor

    The homogeneous equation here is [itex]x^2 y"- 2xy'+ 2=0[/itex]. That's an "equipotential" equation, also called an "Euler type" equation. You can get its general solution by "trying" y= xr as a solution. Also the substitution x= ln(t) changes the equation into a "constant coefficients" equation that is easy to solve.

    The general solution to the homogeneous equation is Cx+ Dx2 where C and D can be any contstants. You can choose them to be -1, certainly. I don't know what you mean by "gave me 0 as the particular solution". y identically equal to 0 certainly doesn't satisfy the equation.

    The Wronskian is a determinant. If you swap two columns, you just multiply the Wronskian by -1. That should do no more than multiply your previous solution by -1.

    Because youdidn't put them in! Your general solution should be Cx+ Dx2+ a particular solution to the entire equation.
     
  4. Sep 14, 2007 #3

    HallsofIvy

    User Avatar
    Science Advisor

    The homogeneous equation here is [itex]x^2 y"- 2xy'+ 2=0[/itex]. That's an "equipotential" equation, also called an "Euler type" equation. You can get its general solution by "trying" y= xr as a solution. Also the substitution x= ln(t) changes the equation into a "constant coefficients" equation that is easy to solve.

    The general solution to the homogeneous equation is Cx+ Dx2 where C and D can be any contstants. You can choose them to be -1, certainly. I don't know what you mean by "gave me 0 as the particular solution". y identically equal to 0 certainly doesn't satisfy the equation.

    The Wronskian is a determinant. If you swap two columns, you just multiply the Wronskian by -1. That should do no more than multiply your previous solution by -1.

    Because youdidn't put them in! Your general solution should be Cx+ Dx2+ a particular solution to the entire equation.
     
  5. Sep 14, 2007 #4
    Thanks!
     
    Last edited: Sep 14, 2007
  6. Sep 14, 2007 #5
    The Wronski keeps confusing me, however.

    W(x^2, x) = -x^2

    W(x, x^2) = x^2

    The particular solution depends on which one of these I choose.
     
  7. Sep 17, 2007 #6
    Or not?
     
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