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Homework Help: A question on undetermined coefficients

  1. Jul 6, 2012 #1
    1. The problem statement, all variables and given/known data
    http://gyazo.com/6c440aa92106f729639c91f6d59dcd89


    3. The attempt at a solution
    My question is why is yp = At^3+Bt^2 +Ct. The reason I ask that is because is see t^2+2t so why wouldnt it be yp=At^2 +Bt +c?
     
  2. jcsd
  3. Jul 6, 2012 #2

    SammyS

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    attachment.php?attachmentid=48917&stc=1&d=1341594524.png

    Because (D3 + 4D)( At3+Bt2 +Ct) = 12At2 + 8Bt + 6A + 4C .

    Whereas, (D3 + 4D)( At2+Bt+C) = 8At + 4B, which is not a quadratic.

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