Cho hàm số \(f(x) = \frac{{\sin 3x}}{3} - \cos x -...

Câu hỏi: Cho hàm số \(f(x) = \frac{{\sin 3x}}{3} - \cos x - \sqrt 3 \left( {\sin x - \frac{{\cos 3x}}{3}} \right)\)Giải phương trình \(f'(x) = 0\)

A. \(\left[ \begin{array}{l}x = \frac{\pi }{{12}} + k\pi \\x =  - \frac{{3\pi }}{8} + k\frac{\pi }{2}\end{array} \right.\left( {k \in Z} \right)\)

B. \(\begin{array}{l}f'(x) = 0\\\left[ \begin{array}{l}x =  - \frac{\pi }{{12}} + k\pi \\x =  - \frac{{3\pi }}{8} + k\frac{\pi }{2}\end{array} \right.\left( {k \in Z} \right)\end{array}\)

C. \(\begin{array}{l}f'(x) = 0\\\left[ \begin{array}{l}x =  - \frac{\pi }{{12}} + k\pi \\x = \frac{{3\pi }}{8} + k\frac{\pi }{2}\end{array} \right.\left( {k \in Z} \right)\end{array}\)

D. \(\begin{array}{l}f'(x) = 0\\\left[ \begin{array}{l}x = \frac{\pi }{{12}} + k\pi \\x = \frac{{3\pi }}{8} + k\frac{\pi }{2}\end{array} \right.\left( {k \in Z} \right)\end{array}\)