Đáp án đúng: C
Giải chi tiết:\(\begin{array}{l}f'\left( x \right) = 2\cos \left( {\frac{\pi }{3} - x} \right).\left( {\cos \left( {\frac{\pi }{3} - x} \right)} \right)' + 2\cos \left( {\frac{\pi }{3} + x} \right).\left( {\cos \left( {\frac{\pi }{3} + x} \right)} \right)'\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + 2\cos \left( {\frac{{2\pi }}{3} - x} \right).\left( {\cos \left( {\frac{{2\pi }}{3} - x} \right)} \right)' + 2\cos \left( {\frac{{2\pi }}{3} + x} \right).\left( {\cos \left( {\frac{{2\pi }}{3} + x} \right)} \right)' - 4\sin x.\left( {\sin x} \right)'\\f'\left( x \right) = - 2\cos \left( {\frac{\pi }{3} - x} \right).\sin \left( {\frac{\pi }{3} - x} \right)\left( {\frac{\pi }{3} - x} \right)' - 2.\cos \left( {\frac{\pi }{3} + x} \right).\sin \left( {\frac{\pi }{3} + x} \right)\left( {\frac{\pi }{3} + x} \right)'\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - 2\cos \left( {\frac{{2\pi }}{3} - x} \right).\sin \left( {\frac{{2\pi }}{3} - x} \right).\left( {\frac{{2\pi }}{3} - x} \right)' - 2\cos \left( {\frac{{2\pi }}{3} + x} \right).\sin \left( {\frac{{2\pi }}{3} + x} \right).\left( {\frac{{2\pi }}{3} + x} \right)' - 4\sin x\cos x\\f'\left( x \right) = 2\sin \left( {\frac{\pi }{3} - x} \right)\cos \left( {\frac{\pi }{3} - x} \right) - 2\sin \left( {\frac{\pi }{3} + x} \right)\cos \left( {\frac{\pi }{3} + x} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + 2\sin \left( {\frac{{2\pi }}{3} - x} \right)\cos \left( {\frac{{2\pi }}{3} - x} \right) - 2\sin \left( {\frac{{2\pi }}{3} + x} \right)\cos \left( {\frac{{2\pi }}{3} + x} \right) - 2\sin 2x\\f'\left( x \right) = \sin \left( {\frac{{2\pi }}{3} - 2x} \right) - \sin \left( {\frac{{2\pi }}{3} + 2x} \right) + \sin \left( {\frac{{4\pi }}{3} - 2x} \right) - \sin \left( {\frac{{4\pi }}{3} + 2x} \right) - 2\sin 2x\\f'\left( x \right) = - 2\cos \frac{{2\pi }}{3}\sin 2x - 2\cos \frac{{4\pi }}{3}\sin 2x - 2\sin 2x\\f'\left( x \right) = \left( { - 2\cos \frac{{2\pi }}{3} - 2\cos \frac{{4\pi }}{3} - 2} \right)\sin 2x\\f'\left( x \right) = \left( { - 2.\left( { - \frac{1}{2}} \right) - 2\left( { - \frac{1}{2}} \right) - 2} \right)\sin 2x\\f'\left( x \right) = 0\end{array}\)
Chọn C.
