Найдите cos2a, если cosA=0.8
Cos2a = cos^2a - sin^2a = cos^2a - (1 - cos^2a) = 2cos^2a - 1 = 2* (0,8)^2 - 1 = 2 * 0.64 - 1 = 1.28 - 1 = 0.28