Which expression is equivalent to cosine 120 degrees ?

Cosine is the x-coordinate on the unit circle. At 120°, the reference angle is 60° and the angle is in Quadrant II, where cosine is negative, so cos 120° = −cos 60° = −1/2. Choice C, cosine 240°, is also −1/2 because 240° is in Quadrant III with a 60° reference angle.

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