Cos2x + 10cosx - 11 = 0

=cos^2x - sin^2 x + 10cos x - 11 (cos^2 x + sin^ x) = cos^2 x - 11cos^2x+10cos x - sin^2 x - 11sin^2 x = 10cos x - 10cos^2 x - 12 sin^2 x=5cos^2 x + 5cos^2 x + 6sin x = 5cos^2 x + 5cos^2 x + 6 (1-cos^2 x) = 5cos^2 x + 5cos^2 x + 6 - 6cos^2 x = cos^2 x - 5cos x - 6=0 ... замена cos x = T ... T^2 - 5T-6=0

T=6

T=-1

Обрат. замена

cosx=6 (не подходит)

cosx=-1. п+2 пk