Question

Найти cos(a), если cos^2(a)-sin^2(a)=1/2  

Answer 1

Cos² a - Sin² a = 1/2
Cos² a -(1 - Cos²a) = 1/2
Cos² a - 1 + Cos² a = 1/2
2Сos² a = 3/2
Cos² a = 3/4
Cos a = √/3/2     или       Cos a = - √3/2