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Undetermined coefficients to find general solution to system

  1. Sep 27, 2007 #1
    I'm having a bit of trouble with a problem here.

    The question is: Use the Method of undetermined coefficients to Find the general solution to th system:

    dx/dt = y + e^t
    dy/dt = -2x + 3y + 1

    I've got the homogenous solution fine, however I'm having a bit of difficulty with the particular solution.

    I used xp = [ ctwe^t + ue^t ] where w was [1,1]^T but i know this trial doesnt include the 1 term and is therefore incorrect.

    Can someone let me know what I'm supposed to do as a trial solution in this case. It's not explained in my notes, and I've looked online but to no avail.

  2. jcsd
  3. Sep 28, 2007 #2


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    What you give will work for the et part- though you really don't need the et- tet is enough. For the "1" you need to add a constant : say
    xp = [ (cte^t+ d)w ]
  4. Sep 29, 2007 #3
    thanks for your help. I still cannot manage to get the solution. Is there anyway you can show some working I'd appreciate it?

    What I did:

    Let xp = [ (cte^t+ d)w ]

    therefore xp (differentiated) = cwe^t + cwte^t + 0

    Then Ax(t) + g(t)
    is cwte^t + [ b + 1 , -2a + 3b ]^T + [ e^t , 1 ]^T

    now we are supposed to equate to get the values of c, a and b but I cannot see how to do this...
  5. Sep 29, 2007 #4


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    What is A? You didn't mention that before. Is that the matrix mutliplying x in your original equation? If so you set that equal to the derivatives on the left. Because et is a solution to the homogenous equation, the terms involving tet will cancel out.
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