Answer:
θ = ±2π/3 +2kπ . . . . . for any integer k
Explanation:
2·cos(θ) +1 = 0
cos(θ) = -1/2 . . . . . subtract 1, divide by 2
The cosine function has the value -1/2 for θ = ±2π/3 and any integer multiple of 2π added to that.
θ = ±2π/3 +2kπ . . . . . for any integer k