Вычислите sin π/7 cos 4π/21+sin 4π/21 cos π/7.

\[\sin\frac{\pi}{7}\cos\frac{4\pi}{21} + \sin\frac{4\pi}{21}\cos\frac{\pi}{7} =\]

\[= \sin\left( \frac{\pi}{7} + \frac{4\pi}{21} \right) = \sin\frac{\pi}{3} = \frac{\sqrt{3}}{2}\]