Вычислите cos π/5 cos 2π/15-sin 2π/15 sin π/5.

\[\cos\frac{\pi}{5}\cos\frac{2\pi}{15} - \sin\frac{2\pi}{15}\sin\frac{\pi}{5} =\]

\[= \cos\left( \frac{\pi}{5} + \frac{2\pi}{15} \right) = \cos\frac{\pi}{3} = \frac{1}{2}\]