Вопрос по алгебре 2 задания 25 баллов

[latex]sina=-\frac{1}{\sqrt3}, \; 180^o\ \textless \ a\ \textless \ 270^o\\cosa=-\sqrt{1-(-\frac{1}{\sqrt3})^2}=-\sqrt{\frac{2}{3}}\\\\sin2a=2sinacosa\\2*(-\frac{1}{\sqrt3})*(-\frac{\sqrt2}{\sqrt3})=\frac{2\sqrt2}{3};\\\\\\\frac{cos2a}{sina*cosa+sin^2a}=ctga-1\\\frac{cos^2a-sin^2a}{sina(cosa+sina)}=\frac{(cosa-sina)(cosa+sina)}{sina(cosa+sina)}=\frac{cosa-sina}{sina}=\\\frac{cosa}{sina}-\frac{sina}{sina}=ctga-1.[/latex]