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Homework Help: 2nd order ODE(nonhomogenous)

  1. Jul 30, 2010 #1
    1. The problem statement, all variables and given/known data

    This was an example the teacher gave us to do in class. Unfortunately, I was a little behind on the notes so was not able to copy down the steps to solving this one. I did, however, get an answer

    problem:

    y'' + y = e^x + x^3, initial conditions y(0)=2, y'(0)=0


    2. Relevant equations



    3. The attempt at a solution

    The answer my prof gave was y=(3/2)cos(x)+(11/2)sin(x)+(1/2)e^(x)+x^3-6x

    I get how he got the complimentary part, but the particular solution I feel I am very close too, and may have just made some slightly wrong assumption as a 'trial' solution.

    what I did for a trial to the particular solution was...

    yp=Ae^x +Bx^3+Cx^2+Dx+E and set this equal too e^x + x^3.

    I found the second derivative to be (Ae^x + 6Bx + 2c). So far I have:

    (Ae^x + 6Bx + 2c) + (Ae^x +Bx^3+Cx^2+Dx+E) = e^x + x^3

    I then gathered terms and solved for coefficients to get A=.5, B=1, C=0, D=-6, and E=0.

    simplifying I ended up with e^x +x^3 = e^x +x^3.... so I did something wrong somewhere I guess.

    I understand everything except how my prof got (1/2)e^(x)+x^3-6x as the particular solution. I even got all the terms he listed as the particular solution, but some of them ended up canceling out so my particular I believe is wrong.
     
  2. jcsd
  3. Jul 30, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    According to your PI, you should get yPI= (1/2)ex+x3-6x.

    Which is correct.
     
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