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Homework Help: DE: Inspection for R(x)=sinx or cosx

  1. Jun 24, 2011 #1
    Can you post solved problems using these conditions? We're on the Advanced Engineering Mathematics and reviewing our Differential Equations. My professor only introduces this topic and didn't went deeper. I'm troubled about the sign convention, e.g.:

    a=3, b=4

    how does yp became positive? What are the formulae used... also for cosine condition?

    Thanks for your help!
  2. jcsd
  3. Jun 24, 2011 #2


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    hi median27! :smile:

    (try using the X2 icon just above the Reply box :wink:)
    it's not a convention, it's just the correct formula …

    D2(sin4x) = -16sin4x

    so (D2 - 9)(sin4x) = (-16 - 9)sin4x = -25sin4x :wink:

    (same for cos4x)
  4. Jun 24, 2011 #3


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    Your formula tells you the particular solution of

    (D2-32)y = +sin 4x

    [tex]y_p = -\frac{\sin 4x}{3^2+4^2}[/tex]
    But that's not the differential equation you have. Yours has a negative sign in front of the forcing term, so you must flip the sign of the particular solution.
  5. Jun 24, 2011 #4
    can you give me the correct formula to use for sine and cosine?

    yp=-sinbx/(a^2+b^2)... is it the standard formula for sine condition?
  6. Jun 24, 2011 #5


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    you can work it out for yourself …

    if y = Ksinbx, when will (D2 - a2)y equal sinbx ? :smile:
  7. Jun 24, 2011 #6
    Alright, i get it. Thanks
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