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what is the second derivatives for Cos²x

$$f(x) = \cos^2 x \\\\f'(x) = 2 \cos x ( - \sin x) = - 2 \cos x \sin x = -\sin 2x\\\\f''(x) = -2\cos 2x\\\\\text{The second derivative of}~ \cos^2 x ~ \text{is}~ -2 \cos 2 x$$