Помогите решить задания, буду очень благодарен)

Решите задачу:

$1)\; \; sin150^\circ =sin(180^\circ -30^\circ )=sin30^\circ=\frac{1}{2}\\\\cos315^\circ =cos(270^\circ +45^\circ )=sin45^\circ=\frac{\sqrt2}{2}\\\\cos\frac{5\pi}{3}=cos(2\pi -\frac{\pi}{3})=cos\frac{\pi}{3}=\frac{1}{2}\\\\sin\frac{4\pi }{3}=sin(\pi +\frac{\pi}{3})=-sin\frac{\pi}{3}=-\frac{\sqrt3}{2}\\\\tg\frac{3\pi}{4}=tg(\pi -\frac{\pi}{4})=-tg\frac{\pi}{4}=-1\\\\tg210^\circ =tg(270^\circ-60^\circ)=ctg60^\circ =\frac{\sqrt3}{3}$

$2)\; \; a)\; \; cosa=\frac{5}{13}\\\\0\ \textless \ a\ \textless \ \frac{\pi}{2}\; \; \to \; \; sina\ \textgreater \ 0\; \; \to \; \; sina=+\sqrt{1-cos^2a}\\\\sina=\sqrt{1-\frac{25}{169}}=\frac{12}{13}\\\\cos2a=cos^2a-sin^2a=\frac{25}{169}-\frac{144}{169}=-\frac{119}{169}$

$b)\; \; sina=\frac{9}{13}\\\\\frac{\pi}{2}\ \textless \ a\ \textless \ \pi \; \; \to \; \; cosa\ \textless \ 0\; \; \to \; \; cosa=-\sqrt{1-sin^2a}\\\\cosa=-\sqrt{1- \frac{81}{169} }= -\frac{\sqrt{88}}{13} \\\\sin2a=2sina\cdot cosa=2\cdot \frac{9}{13}\cdot (-\frac{\sqrt{88}}{13})=-\frac{18\sqrt{88}}{169}=-\frac{36\sqrt{22}}{169}$