Question

СРОЧНО!Найдите cos2α , если sinα = -√2/3и α ϵ (3/2 π;2π)

Answer 1

$cos2\alpha=cos^2\alpha-sin^2\alpha =(1-sin^2\alpha )-sin^2\alpha=1-2sin^2\alpha=\\=1-2*(-\frac{\sqrt{2} }{3} )^2=1-2*\frac{2}{9} =1-\frac{4}{9}=\frac{5}{9}$