Solve the equation cos (3θ +120°) = √(3 /2 for 0° ≤ θ≤ 180°

Answer 1

cos(x) = √3/2 if x = 30° or 330°
So,
3θ+120 = 30, 330, 390, 690, ...
θ = -30, 70, 90, 190, ...

So, pick the values in your domain.