2Сos^2x + 2SinxCosx - 8(Cos^2x - Sin^2x) = 0
2Cos^2x + 2SinxCosx - 8Cos^2x + 8Sin^2x = 0
8Sin^2x + 2SinxCosx -6Cos^2x = 0 | : Cos^2x
8tg^2x +2tgx -6 = 0
4tg^2x + tgx -3 = 0
Решаем как квадратное.
D=49
a)tgx = (-1 +7)/8 = 6/8 =3/4
x = arctg(3/4) +πk. k Є Z
б) tg x = (-1-8)/8 = -1
x = - π/4 + πk. k Є Z