Which expression is equivalent to cos 120°?

cos 120° = - 1/2

Step-by-step explanation:

cos 120° = (adjacent side) / (hypotenuse.

Here the adjacent side (the side next to the central angle in this 30-60-90 triangle) is - 1. Then the hypotenuse is 2 and the opposite side is √3.

Thus, cos 120° = - 1/2