[tex]\sin^2a+\cos^2a=1\Leftrightarrow \sin^2a=1-\cos^2a=1-\left ( -\frac{4}{5} \right )^2=\frac{9}{25}\Rightarrow \\\Rightarrow \sin a=\pm \sqrt{\frac{9}{25}}=\pm \frac{3}{5}[/tex]