Вычислите: 1. cos 5П/3 2. sin345° 3. tg(-240°)

Решите задачу:

$1.\; cos\frac{5\pi}{3}=cos(\frac{6\pi}{3}-\frac{\pi}{3})=cos(2\pi-\frac{\pi}{3})=cos\frac{\pi}{3}=\frac{1}{2};\\\\ 2. \; sin345^o=sin(360^o-15^o)=-sin15^o=-sin(45^o-30^o)=\\=-(sin45^ocos30^o-cos45^osin30^o)=-(\frac{\sqrt2}{2}*\frac{\sqrt3}{2}-\frac{1}{2}*\frac{1}{2})=\frac{1-\sqrt6}{4};\\\\ 3.\; tg(-240^o)=-tg(270^o-30^o)=-tg30^o=-\frac{\sqrt3}{3}.$