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Diff Eq. - swapped coefficients help

  1. Apr 2, 2008 #1
    1. The problem statement, all variables and given/known data

    In an L-R-C circuit, L = 1, R = 2, C = 0.25, E(t) = 50 cos(t)

    Find the steady state solution

    2. Relevant equations

    3. The attempt at a solution

    [tex] L \frac {di(t)}{dt} + R \frac {dq(t)}{dt} + \frac {q}{C} = 50cos(t) [/tex]

    [tex] \frac {di(t)}{dt} + 2 \frac {dq(t)}{dt} + 4q = 50cos(t) [/tex]

    [tex] q(t) = e^{-t}(c1cos(sqrt(12)t + c2sin(sqrt(12)t) [/tex]

    next annihilate 50*cos(t)
    [tex] (D^2 +1) (D^2 + 2D + 4) = (D^2 +1)50cos(t) [/tex]

    [tex] roots = +/- i[/tex]

    [tex] qp(t) = Acos(t) + Bsin(t)[/tex]

    [tex] [-Acos(t) - Bsin(t)] - 2Asin(t) + 2Bcos(t) + 4Acos(t) + 4Bsin(t) = 50cos(t) [/tex]

    [tex] cos(t)[-A+2B+4A] + sin(t)[-B-2A+4B] = 50cos(t) [/tex]

    [tex] 3A+2B = 50 [/tex]

    [tex] 3B-2A = 0[/tex]

    [tex] A = \frac {150}{13} [/tex]

    [tex] B = \frac {100}{13} [/tex]

    [tex] qp(t) = \frac {150}{13}cos(t) + \frac {100}{13}sin(t) [/tex]

    The book gets
    [tex] qp(t) = \frac {100}{13}cos(t) - \frac {150}{13}sin(t) [/tex]

    I'm off with swapped coefficients and the sign.

    Do I have something crossed?
    Where is my error?

    Last edited: Apr 2, 2008
  2. jcsd
  3. Apr 2, 2008 #2


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    Staff Emeritus
    Science Advisor

    Are we to assume that i= dq/dt?

    How did you get that? If you took i= dq/dt so that di/dt= d2q/dt2, then characteristic equation is r2+ 2r+ 4= (r+2)2= 0 and the general solution (to the associated homogeneous equation) is
    [tex]q(tt)= c1e^{-2t}+ c2 te^{-2t}[/tex]

  4. Apr 2, 2008 #3
    You are correct on the first (homogeneous solution) - I had an obvious error in my work. I didn't recognize the obvious solution and made a mistake in factoring.

    (However, I don't need that solution to get the steady state) I need the particular qp(t)

    the [tex] (D^2 +1) (D^2 + 2D + 4) = (D^2 +1)50cos(t) [/tex]

    the second term is the solution to the homogeneous - as you pointed out [tex]q(tt)= c1e^{-2t}+ c2 te^{-2t}[/tex]

    The [tex] (D^2 +1) [/tex]

    [tex] roots = +/- i[/tex]

    with all the equating coefficients is where my error is.


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