$$\left. \begin{array} { l } { ( \cos 2 A + \cos 2 B ) ^ { 2 } + ( \sin 2 A - \sin 2 B ) ^ { 2 } = 4 \cos ^ { 2 } ( A + B ) } \\ { ( 2 \cos \theta + 1 ) ( 2 \cos \theta - 1 ) ( 2 \cos 2 \theta - 1 ) = 2 \cos 4 \theta + 1 } \end{array} \right.$$
$\theta \in \mathrm{R}$
$A\in \mathrm{R}$
$B\in \mathrm{R}$
