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Homework Help: Reduce Order (diff eq)

  1. Mar 7, 2008 #1
    [SOLVED] Reduce Order (diff eq)

    nevermind i got it, cliffsnotes ftw

    1. The problem statement, all variables and given/known data
    Solve the differential equation using the reduction of order method.
    [tex]t^2 y'' - 4ty' + 6y = 0[/tex]
    [tex]t > 0[/tex]
    [tex]y_1 (t) = t^2[/tex]
    2. Relevant equations



    3. The attempt at a solution
    Well The first thing I do is
    [tex]y(t) = v(t) t^2[/tex]
    Then I find y' and y'' and plug into the original diff eq and get
    [tex]t^4 v'' = 0[/tex]
    Which I'll assume is correct.

    But now I'm really not sure what to do with that. I could do integration by parts? but that doesn't seem to lead anywhere. How do I get from there to t^3 (the other solution)?
     
    Last edited: Mar 7, 2008
  2. jcsd
  3. Mar 8, 2008 #2

    HallsofIvy

    User Avatar
    Science Advisor

    I'm glad you solved it yourself- you would have felt embarassed if someone else had pointed out the obvious!

    You divide both sides by t4 to get v"= 0. Since v" is 0, v'= C, a constant. Then v= Ct+ D, another constant. Since y= vt2, y= Ct3+ Dt2 is the general solution. By the way, we can divide by t4 only if t is not 0. The equation is singular at t= 0- the existance and uniqueness theorem does not apply if we are given intial conditions at t= 0. For example, there is no solution if we are given y(0)= 1, y'(0)= 0.
     
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