To draw theta=-120° in the unit circle, we move clockwise 90°, then another 30°.
This means that we pass the fourth quadrant and stop in the third quadrant.
The reference angle of an angle theta in the third quadrant, is theta + 180°.
So the reference angle of -120° is -120°+180°=60°.
360° are 2π Radians
then 60° are 1/6 of 2π Radians = π/3 Radians
Answer: Reference angle is π/3 Radians