Question

Известно, что cos a=7/25 , 3П/2<a<2П. Найти cos2a

Answer 1

cos^2a=49/625

cos2a=2cos^2-1=98/625-1=-527/625

Answer 2

$cos2alpha=2cos^2alpha-1=2(frac{7}{25})^2-1=frac{98-625}{625}=-frac{527}{625}$