Question

найдите sina a, если cos a = - 0,8  П/2 < a < П 

Answer 1

$sin a > 0, a in (frac{pi}{2}, pi)\\ sin a = sqrt{1 - cos^2a}\\ sin a = sqrt{1 - (-0.8)^2}\\ sin a = sqrt{1 - 0.64}\\ sin a = sqrt{0.36}\\ boxed{sin a = 0.6}$