$$\left. \begin{array} { l } { 4 ( \cos ^ { 6 } \theta - \sin ^ { 6 } \theta ) = \cos ^ { 2 } 2 \theta + 3 \cos ^ { 2 } \theta } \\ { 3 ( \sin x - \cos x ) ^ { 4 } + 6 ( \sin x + \cos x ) ^ { 2 } + 4 ( \sin ^ { 6 } x + \cos ^ { 6 } x ) = 13 } \\ { \sqrt \right.$$
