Question

6-5cos^2a если sin a = 2/корень10

Answer 1

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

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