We have to differentiate f(x) = x*cos 2x
f'(x) = x'*cos 2x + x*(cos 2x)'
f'(x) = cos 2x + x*(-sin 2x)*2
f'(x) = cos 2x - 2x*(sin 2x)
The required derivative of f(x) = x*cos 2x is f'(x) = cos 2x - 2x*(sin 2x)
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