Giải các phương trình lượng giác sau

a) \(\cos 2x = \cos \frac{\pi }{3}\)\( \Leftrightarrow \left[ \begin{array}{l}2x = \frac{\pi }{3} + k2\pi \\2x = - \frac{\pi }{3} + k2\pi \end{array} \right.,\,\,\,k \in \mathbb{Z}\)\( \Leftrightarrow \left[ \begin{array}{l}x = \frac{\pi }{6} + k\pi \\x = - \frac{\pi }{6} + k\pi \end{array} \right.,\,\,\,k \in \mathbb{Z}\)

              b) \(\sin (x + \frac{\pi }{3}) = \frac{{\sqrt 3 }}{2}\).

              \(\sin (x + \frac{\pi }{3}) = \frac{{\sqrt 3 }}{2} \Leftrightarrow \sin (x + \frac{\pi }{3}) = \sin \frac{\pi }{3}\)

              \( \Leftrightarrow \left[ \begin{array}{l}x + \frac{\pi }{3} = \frac{\pi }{3} + k2\pi \\x + \frac{\pi }{3} = \frac{{2\pi }}{3} + k2\pi \end{array} \right.,\,\,\,k \in \mathbb{Z} \Leftrightarrow \left[ \begin{array}{l}x = k2\pi \\x = \frac{\pi }{3} + k2\pi \end{array} \right.,\,\,\,k \in \mathbb{Z}\)