\(\frac{\pi}{2}< a< \pi\Rightarrow cosa< 0\Rightarrow cosa=-\sqrt{1-sin^2a}=-\frac{\sqrt{7}}{4}\)
\(tana=\frac{sina}{cosa}=-\frac{3\sqrt{7}}{7}\) ; \(cota=\frac{1}{tana}=-\frac{\sqrt{7}}{3}\)
\(A=\frac{-\frac{6\sqrt{7}}{7}+\sqrt{7}}{-\frac{\sqrt{7}}{4}+\frac{3\sqrt{7}}{7}}=\frac{4}{5}\)