Which expression is equivalent to $\cos 120°$?

• A. $\cos 300°$
• B. $\cos 240°$
• C. $\cos 420°$
• D.

Answer: (C)

Verified step by step guidance

1. Recall that the cosine function has a period of 360°, meaning that for any angle \( \theta \), \( \cos(\theta) = \cos(\theta + 360°) \).
2. To find an expression equivalent to \( \cos 120° \), consider adding or subtracting multiples of 360° to 120° to get other angles with the same cosine value.
3. Add 360° to 120° to get \( 120° + 360° = 420° \), so \( \cos 120° = \cos 420° \).
4. Check the other given angles: 300° and 240° are not related to 120° by adding or subtracting 360°, so their cosine values are generally different from \( \cos 120° \).
5. Therefore, the expression \( \cos 420° \) is equivalent to \( \cos 120° \) because of the periodicity of the cosine function.