#andy
\[\begin{array}{l}
\cos x - \cos 2x = - 2\\
\Leftrightarrow \cos x - 2{\cos ^2}x + 1 = - 1\\
\Leftrightarrow - 2{\cos ^2}x + \cos x + 3 = 0\\
\Leftrightarrow \{ \begin{array}{*{20}{c}}
{\cos x = \frac{3}{2}(l)}\\
{\cos x = - 1(1/n)}
\end{array}\\
\Leftrightarrow x = \pi + k2\pi (k \in z)
\end{array}\]
