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General solution of second order ODE

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data

    Find the general solution.

    2. Relevant equations


    3. The attempt at a solution

    Characteristic equation would be:
    m2 + 1 = 0

    So,m2 = -1

    Therefore, m = i or m = -i.

    Complementary function would be : Asinx+Bcosx where,A and B are constants respectively.

    If I write the particular integral as (Cx2+Dx+E)sin2x + (Px2+Qx+R)cos2x
    Then,it would be very tedious to solve.

    Is there any alternative way like writing sin2x as Imaginary part of ei2x and then solving the particular integral?
  2. jcsd
  3. Jan 18, 2015 #2


    Staff: Mentor

    But not that tedious. All you have to do is take the first and second derivatives, and substitute them into your DE to determine the six constants. It might be there is another way, but this is how I would do the problem.
  4. Jan 21, 2015 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Another approach is Laplace transform:

    For the right-hand term,
    if f(x) ↔ F(s), then
    x2f(x) ↔ F''(s)
    and f(x) = sin(2x) and F(s) = L{sin(2x)}.

    You can incorporate the two initial conditions on y also in the usual manner if they're not identically zero.
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