Answer:
You didn't add the range. I'll use (0<x<2π)
cosx = -1/2
x=-cos-¹(1/2)
x= -60°
Cosine is negative in the second and 3rd quadrants
180-x ---- Second quadrant
x+180 ----- 3rd quadrant
So
Our answer
180-(-60)=240° or 4π/3 second quad
180+(-60) = 120° or 2π/3---- 3rd quad
