Question

cos2a,если COSA=1/корень из 5

Answer 1

$cosa= frac{1}{sqrt5} \\sin^2a=1-cos^2a=1-frac{1}{5}=frac{4}{5}\\cos2a=cos^2a-sin^2a=frac{1}{5}-frac{4}{5}=-frac{3}{5}$