2Cos²x = 8Cos2x - Sin2x
2Cos²x - 8(Cos²x - Sin²x) + 2SinxCosx = 0
2Cos²x - 8Cos²x + 8Sin²x + 2SinxCosx = 0
-6Cos²x +8 Sin²x + 2SinxCosx = 0
3Cos²x - 4Sin²x - SinxCosx =0 | : Cos²x≠ 0
3 -4tg²x -tgx = 0
tgx = y
4y² + y -3 = 0
D = 49
y₁ = -1+7)/8 = 6/8 = 3/4
у₂ = (-1 -7)/8 = -1
а) у = 3/4
tgx = 3/4
x = arctg3/4 + πk , k ∈ Z
б) y = -1
tgx = -1
x = -π/4 + πn , n ∈Z