Giải phương trình cos3x = cos pi/15

\(\cos 3x = \cos \frac{\pi }{{15}}\)

 \( \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{3x = \frac{\pi }{{15}} + k2\pi }\\{3x = - \frac{\pi }{{15}} + k2\pi }\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{x = \frac{\pi }{{15}}.\frac{1}{3} + k2\pi }\\{x = - \frac{\pi }{{15}}.\frac{1}{3} + k2\pi }\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{x = \frac{\pi }{{45}} + k2\pi }\\{x = - \frac{\pi }{{45}} + k2\pi }\end{array}} \right.\left( {k \in \mathbb{Z}} \right)\) .