Вычислите (cos 7π/12-sin 7π/12)(cos 7π/12+sin 7π/12).

\[= \cos^{2}\frac{7\pi}{12} - \sin^{2}\frac{7\pi}{12} =\]

\[= \cos\left( 2 \cdot \frac{7\pi}{12} \right) = \cos\frac{7\pi}{6} =\]

\[= \cos\left( \pi + \frac{\pi}{6} \right) = - \cos\frac{\pi}{6} = - \frac{\sqrt{3}}{2}\]