Question

найдите sina если Cosa=-4/5
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Answer 1

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