The solution to the problem is as follows:
f(x) = 3cos^2(x)f'(x) = 3*2cos(x)*d(cos(x))/dxf'(x) = 6cos(x)*(-sin(x))f'(x) = -6sin(x)cos(x), since sin(2x) = 2sin(x)cos(x)f'(x) = -3sin(2x)
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