Сравните значения выражений: a) cos 13pi/7 и sin 180° б) cos 2pi/3 и 2pi/3

Решите задачу:

$1)\; \; cos\frac{13\pi }{7}=cos(2\pi -\frac{\pi}{7})=cos\frac{\pi}{7}\\\\sin180^\circ =0\\\\cos\frac{\pi}{7}\ \textgreater \ 0\; \; ,\; tak\; kak\; \; 0\ \textless \ \frac{\pi}{7}\ \textless \ \frac{\pi}{2}\\\\2)\; \; cos\frac{2\pi}{3}\ \textless \ 0\; ,\; \; tak\; kak\; \; \frac{\pi}{2}\ \textless \ \frac{2\pi}{3}\ \textless \ \pi \; ,\; a\; \; \frac{2\pi}{3}\ \textgreater \ 0\; \to \\\\cos\frac{2\pi}{3}\ \textless \ \frac{2\pi}{3}$