Пожалуйста решите кроме 4. Срочно

Решите задачу:

$1)\quad sin15\cdot sin45+cos15\cdot cos45=cos(45-15)=cos30=\frac{\sqrt3}{2}\\\\2)\quad sina=-\frac{2}{5}\\\\ a\in (\pi ,\frac{3\pi}{2})\quad \Rightarrow \quad sina\ \textless \ 0,\; cosa\ \textless \ 0,\; tga\ \textgreater \ 0,\; ctga\ \textgreater \ 0\\\\cosa=-\sqrt{1-sin^2a}=-\sqrt{1-\frac{4}{25}}=-\sqrt{\frac{21}{25}}=-\frac{\sqrt{21}}{5}\\\\1+tg^2a=\frac{1}{cos^2a}\quad \Rightarrow \quad tga=\pm \sqrt{\frac{1}{cos^2a}-1}\\\\tga=+\sqrt{\frac{1}{\frac{21}{25}}-1}=\sqrt{\frac{4}{21}}=\frac{2}{\sqrt{21}}\\\\ctga=\frac{1}{tga}=\frac{\sqrt{21}}{2}$

$3)\quad \frac{a}{6\sqrt3} =cos30\; ;\; \; \frac{a}{6\sqrt3} = \frac{\sqrt3}{2} \\\\a= \frac{6\sqrt3\cdot \sqrt3}{2} =9\\\\ \frac{b}{6\sqrt3} = sin30\; ;\; \; \frac{b}{6\sqrt3} = \frac{1}{2}\\\\b= \frac{6\sqrt3}{2} =3\sqrt3\\\\a\ \textgreater \ b\\\\S= \frac{1}{2}ab= \frac{1}{2} \cdot 9\cdot 3\sqrt3= \frac{27\sqrt3}{2}$