Sin7пи/5-sin17пи/10 помогитее

[latex]sin \frac{7 \pi }{5} -sin\frac{17 \pi }{10}= sin \frac{5 \pi +2\pi}{5} -sin\frac{20 \pi-3\pi }{10}= \\ \\ =sin (\pi +\frac{2\pi}{5}) -sin(2\pi -\frac{3\pi }{10})= -sin\frac{2\pi}{5}-(-sin\frac{3\pi }{10}))= \\ \\ =sin\frac{3\pi }{10}-sin\frac{2\pi}{5}=2sin \frac{\frac{3\pi }{10}-\frac{2\pi}{5}}{2} cos\frac{\frac{3\pi }{10}+\frac{2\pi}{5}}{2}=2sin \frac{-\pi}{20}cos \frac{7\pi}{20}= \\ \\ =-2 sin \frac{\pi}{20}cos \frac{7\pi}{20}[/latex]