Giải phương trình cos 2x = cos 2 pi/ 3 .

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

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