1) (cos 2a+cos(п/2))/2 + (sin^2 a)/2 = (1/2)*cos(a).
2) cos 2a- (1/2)*(cos 2a+cos 4a) = (1/2)*(cos 2a-cos 4a) = sin 3a*sin a.
1) (cos 2a+cos(п/2))/2 + (sin^2 a)/2 = (1/2)*cos(a).
2) cos 2a- (1/2)*(cos 2a+cos 4a) = (1/2)*(cos 2a-cos 4a) = sin 3a*sin a.