Найдите косинусы углов: 1) -17п\4 2)17п\6 3)-10п\3 с решением плиз)

Решите задачу:

$Cos(-\frac{17\pi}{4})=Cos(4\pi +\frac{\pi}{4})=Cos\frac{\pi}{4}=\frac{\sqrt{2}}{2} \\\\Cos\frac{17\pi}{6}=Cos(2\pi+\frac{5\pi}{6})=Cos\frac{5\pi}{6}=Cos(\pi-\frac{\pi}{6})=-Cos\frac{\pi}{6}=-\frac{\sqrt{3}}{2}\\\\Cos(-\frac{10\pi}{3} )=Cos(2\pi+\frac{4\pi}{3} )=Cos\frac{4\pi}{3}=Cos(\pi +\frac{\pi}{3} )=-Cos\frac{\pi}{3} =-\frac{1}{2}$