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Homework Help: Diff. Eq. : Undetermined Coefficents

  1. Oct 13, 2005 #1
    I have 2 problems that I'm not getting the right answer to:

    y"+4y=3sin(2t)

    I chose my guess to be: y=A[tsin(2t)]+B[tcos2t]
    with this guess I'm getting only part of the answer right

    I also tred this guess: y= A[t^2(sin(2t)]+ B[t^2(cos(2t)] + C[tsin(2t)]+D[tcos2t]

    I got the same answer as above + some other weird answers


    y"-y'-2y=(e^t+e^(-t))/2

    I made this guess: Ae^t+Bte^(-t)

    I got my answer as -2A=1/2 and -3B=1/2

    However these were also wrong. Anything wrong with the guesses?
     
  2. jcsd
  3. Oct 13, 2005 #2

    Pyrrhus

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    What were the fundamental (complimentary) set of solutions for each ODE?
     
  4. Oct 13, 2005 #3
    Answers...

    The first one: Asin(2t)+Bcos(2t)-(1/8)sin(2t)-(3/4)tcost(2t)
    The second one : Ae^-t+Be^-2t+(1/6)te^(2t)+(1/8)e^(-2t)
     
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