Final answer:
To find cos 120°, consider that it's in the second quadrant where cosine is negative, and its reference angle is 60°, leading to the conclusion that cos 120° equals -½.
Step-by-step explanation:
To find the exact value of cos 120°, we can use the fact that 120° is an angle in the second quadrant where cosine values are negative, and also apply the reference angle concept. The reference angle for 120° is 60°, since it is 180° - 120°. Since cos 60° is ½, and considering that cosine of an angle in the second quadrant is negative, we conclude that cos 120° is -½.