Вычислите cos 3π/10 cos π/20+sin π/20 sin 3π/10.

\[\cos\frac{3\pi}{10}\cos\frac{\pi}{20} + \sin\frac{\pi}{20}\sin\frac{3\pi}{10} =\]

\[= \cos\left( \frac{3\pi}{10} - \frac{\pi}{20} \right) = \cos\frac{\pi}{4} = \frac{\sqrt{2}}{2}\]