Question

Найдите sin a если П

Answer 1

$cos(2\alpha)=cos^2\alpha-sin^2\alpha=1-sin^2\alpha-sin^2\alpha=1-2sin^2\alpha\\\\1-2sin^2\alpha=0,82\\\\2sin^2\alpha=1-0,82\\\\2sin^2\alpha=0,18\ \ \ |:2\\\\sin^2\alpha=0,09\\\\sin\alpha=-\sqrt{0,09}\\\\sin\alpha=-0,3$