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Homework Help: 3rd order differential equation

  1. Jul 18, 2012 #1
    1. The problem statement, all variables and given/known data
    I took differential eq-n class more than a year ago and I am having problem solving this problem.
    5(d^3y/dt^3)+3(d^2y/dt^2)+7(dy/dt)+y = 4(d^2f/dt^2), where f(t)=cos(t)-2sin(t)

    2. Relevant equations

    3. The attempt at a solution
    First thing I did was to take the second derivative of f(t) and multiply it by 4 for the right side of the equation, which turns out to be 4(-cos(t)+2sin(t))=-4cos(t)+8sin(t).
    For the left hand side, I take its characteristic equation 5y^3+3y^2+7y+1=0 and tried to solve for the roots of the cubic equation, but turns out the roots are not whole & real numbers:
    t1 = -0.15009708847612485
    t2 = 0.22495145576193756+1.1321959748355932i
    t3 = 0.22495145576193756-1.1321959748355932i
    Then I am not sure how to proceed... or did I use a wrong approach? If not then how should I proceed the problem with what method? (undetermined coefficients?) Not sure if this helps, this differential equation describes a circuit with y(t) as the output and f(t) as the input. I was asked to find the steady-state output of the circuit. So I figure I should take the limit of the solution of the differential equation as t approach infinity.

    Any help will be appreciated!
  2. jcsd
  3. Jul 18, 2012 #2


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    Homework Helper

    In the stationary state, the output will be of the same frequency as the input signal. The real part of all roots are negative, you made a mistake when copying them. The solutions of the homogeneous equation tend to zero with time. Assume the stationary solution in form of Acos(t)+Bsin(t) and solve for A, B.

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