#andy
\[\begin{array}{l}
\cos (x - \frac{{2\pi }}{5}) = \cos 2x\\
\Rightarrow \{ \begin{array}{*{20}{c}}
{x - \frac{{2\pi }}{5} = 2x + k2\pi }\\
{x - \frac{{2\pi }}{5} = - 2x + k2\pi }
\end{array}\\
\Rightarrow \{ \begin{array}{*{20}{c}}
{x = - \frac{{2\pi }}{5} - k2\pi }\\
{x = \frac{{2\pi }}{{15}} + \frac{{k2\pi }}{3}}
\end{array}
\end{array}\]
