Cos^2 x + 2sin x*cos x -3sin^2 x = 0
Делим всё на -cos^2 x
3tg^2 x - 2tg x - 1 = 0
(tg x - 1)(3tg x + 1) = 0
tg x = 1; x = Π/4 + Π*k
tg x = -1/3; x = -arctg(1/3)+Π*n
sin^2 x + 2cos^2 x - cos x*sin x = 1
sin^2 x + cos^2 x + cos^2 x - cos x*sin x = 1
1 + cos^2 x - cos x*sin x = 1
cos x*(cos x - sin x) = 0
cos x = 0; x = Π/2 + Π*k
cos x = sin x
tg x = 1; x = Π/4 + Π*n