$$\left. \begin{array} { l } { \cos 2 x - \cos x - 2 = 0 } \\ { \log \sin ^ { 2 } x + \cos 2 x - 1 = 0 } \\ { \cos 4 x - \sqrt { 2 } \cos 2 x = \frac { 1 } { 2 } } \end{array} \right.$$
