Вычислите (cos 5π/12-sin 5π/12)(cos 5π/12+sin 5π/12).

\[= \cos^{2}\frac{5\pi}{12} - \sin^{2}\frac{5\pi}{12} =\]

\[= \cos\left( 2 \cdot \frac{5\pi}{12} \right) = \cos\frac{5\pi}{6} = - \frac{\sqrt{3}}{2}\]