Question

Найдите cos^2a ,если cos2a=2/3

Answer 1

$cos2\alpha=2cos^2\alpha-1\\\\\frac{2}{3}=2cos^2\alpha-1\\\\2cos^2\alpha=\frac{2}{3}+1\\\\2cos^2\alpha=\frac{5}{3}\ /:2\\\\cos^2\alpha=\frac{5}{6}$