Giải phương trình: cos 3x + cos pi/5 = 0 .

\(\cos 3x =  - \cos \frac{\pi }{5}\)\[ \Leftrightarrow \cos 3x = \cos \frac{{4\pi }}{5}\]

 \[ \Leftrightarrow \left[ \begin{array}{l}3x = \frac{{4\pi }}{5} + k2\pi \\3x =  - \frac{{4\pi }}{5} + k2\pi ,\end{array} \right.\left( {k \in \mathbb{Z}} \right)\]\[ \Leftrightarrow \left[ \begin{array}{l}x = \frac{{4\pi }}{{15}} + \frac{{k2\pi }}{3}\\x =  - \frac{{4\pi }}{{15}} + \frac{{k2\pi }}{3},\end{array} \right.\left( {k \in \mathbb{Z}} \right)\].