Cos π знаменатель10 -Cos π знаменатель 20

Решите задачу:

$cos\frac{\pi}{10}-cos\frac{\pi}{20}=-2sin({\frac{1}{2}(\frac{\pi}{10}+\frac{\pi}{20}))sin(\frac{1}{2}(\frac{\pi}{10}-\frac{\pi}{20}))=-2sin\frac{3\pi}{40}sin\frac{\pi}{40}$