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Homework Help: Few questions about solving DE

  1. Nov 18, 2012 #1


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    1. The problem statement, all variables and given/known data

    See the image here : http://gyazo.com/de17b74fd351f26e2e361ae5d975b390

    2. Relevant equations

    3. The attempt at a solution

    I'm a bit rusty at solving DEs since I was quite far ahead of the rest of the class and haven't practiced much. I'm just wondering if my thoughts about these are correct.

    For the first one, I should solve the homogeneous system for a solution yc and then use the method of undetermined coefficients to solve the non-homogeneous one. I believe my particular solution guess should be Y = Ax + b.

    Same process for the second one, except my guess should be Y = Asin(2t) + Bcos(2t) + Ce-4t.

    For the third one, I'm pretty sure I solve the homogeneous system and then use the method of variation of parameters... or I could use something I figured out on my own time I'm pretty sure. Couldn't I just figure out a particular solution here by using :

    [itex]Y(t) = \sum_{m=1}^{n} y_m(t) \int \frac{g(t) W_m(t)}{W(t)}dt[/itex] where ym(t) is one of our known solutions to the homogeneous system, g(t) is our equation on the right hand side of the DE, Wm(t) is the determinant of W obtained by replacing the mth column by (0, 0, 0, ..., 1) ( Cramer's Rule ), and W(t) is the wronskian of my known solutions to the homogeneous equation.

    The last one looks ugly. Even though I can solve it using undetermined coefficients, I can tell it's going to get quite messy because if I do, my guess will be :

    Y = (At5 + Bt4 + Ct3 + Dt2 + Et + F)e-t

    Wouldn't abusing my formula from above again be more desirable here as it would probably reduce the algebra involved?

    Thanks for the help in advance.
    Last edited: Nov 18, 2012
  2. jcsd
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