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d2y/dx2+9y=cos2x+sin2x

  • math -

    y" + 9y = cos2x+sin2x

    You know that

    y"+9y=0
    has solutions
    y = a cos3x + b sin3x

    so the solution here is

    y = a cos 3x + b sin 3x + (cos2x+sin2x)/5

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