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Posted by aaa on Wednesday, July 2, 2014 at 1:57am.

d2y/dx2+9y=cos2x+sin2x

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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