math
posted by aaa .
d2y/dx2+9y=cos2x+sin2x

math 
Steve
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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