Cos квадрат a - cos 2 a =?

Question

Cos квадрат a - cos 2 a =?

Answer 1

Cos^2 a - cos2a = cos^2a - (cos^2a  - sin^2a) =
= cos^2a - cos^2a + sin^2a =  sin^2a

Запомни!
cos2a = cos^2a - sin^2a = 2cos^2a - 1 = 1 - 2sin^2a