sin(pi + x) = -sin x
sin^2 (pi + x) = sin^2 x
cos(pi - x) = -cos x
Подставляем
6sin^2 x + (-sin x)(-cos x) - cos^2 x = 0
6sin^2 x + sin x*cos x - cos^2 x = 0
Делим все на
cos^2 x
6tg^2 x + tg x - 1 = 0
(2tg x + 1)(3tg x - 1) = 0
tg x = -1/2; x1 = -arctg (1/2) + pi*k
tg x = 1/3; x2 = arctg (1/3) + pi*n