Вычислите cosa, если sina=3/5, 90°<a<180°.

\[\sin\alpha = \frac{3}{5};\ \ \]

\[90{^\circ} < \alpha < 180{^\circ} \Longrightarrow \cos\alpha < 0\]

\[\cos\alpha = \sqrt{1 - \sin^{2}\alpha} =\]

\[= \sqrt{1 - \left( \frac{3}{5} \right)^{2}} = \sqrt{1 - \frac{9}{25}} =\]

\[= \sqrt{\frac{16}{25}} = \left| \frac{4}{6} \right| = - \frac{4}{5}\ \]