Question

cos 2x + cos x + 1 = 0

Answer 1

cos2x = 2cos²x - 1
2cos²x - 1 + cosx + 1 = 0
2cos²x + cosx = 0
cosx(2cosx + 1) = 0
cosx = 0 => x = π/2 + kπ, k ∈ Z
ou x = 2π/3 + 2kπ, k ∈ Z ou
x = 4π/3 + 2kπ, k ∈ Z