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Math Help - 4th order DE

  1. #1
    Junior Member
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    4th order DE

    Please help me check my solution, because I'm not sure that it is correct or not.

    \frac{d^4y}{dx^4}-7\frac{d^2y}{dx^2}-18y=0

    m^4e^{mx}-7m^2e^{mx}-18e^{mx} = 0

    (m^4-7m^2-18)(e^{mx}) = 0

    (m-3)(m+3)(m^2+2) = 0

    m = 3, -3, \sqrt{2}i, -\sqrt{2}i

    Then, y=c_1e^{3x}+c_2e^{-3x}+c_3cos(\sqrt{2}x)+c_4sin(\sqrt{2}x)+c_5cos(\sq  rt{2}x)-c_6sin(\sqrt{2}x)
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  2. #2
    Super Member Random Variable's Avatar
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    remove the last two terms from your solution and then it's correct
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