Question

Найдите cos^2a ,если cos2a=2/3

Answer 1

$cos2alpha=2cos^2alpha-1\\frac{2}{3}=2cos^2alpha-1\\2cos^2alpha=frac{2}{3}+1\\2cos^2alpha=frac{5}{3} /:2\\cos^2alpha=frac{5}{6}$