Вычислите sin 13π/42-sin π/42-cos 10π/21-cos 4π/21.

\[= 2\sin\frac{\pi}{7} \cdot \frac{\sqrt{3}}{2} - 2 \cdot \frac{\sqrt{3}}{2} \cdot \sin\frac{\pi}{7} =\]

\[= \sqrt{3}\sin\frac{\pi}{7} - \sqrt{3}\sin\frac{\pi}{7} = 0\]