Question

6-5cos^2a если sin a = 2/корень10

Answer 1

$6-5\cos^2 \alpha =6-5(1-\sin^2\alpha )=6-5+5\sin^2\alpha =1+5\sin^2\alpha$

Подставим $\sin \alpha = \dfrac{2}{ \sqrt{10} }$, имеем $1+5\sin^2\alpha =1+5\cdot\bigg(\dfrac{2}{ \sqrt{10} }\bigg)^2=3$