Find the value of $\cos (- 120 °)$

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$- \frac{1}{3}$

$- \frac{1}{2}$

answer is B.

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Detailed Solution

The trigonometric function cos 120 represents the cosine of 120 degrees. To find the value of cos 120, we can analyze its position on the unit circle.

Step-by-Step Explanation

The angle 120° lies in the second quadrant of the unit circle, where the cosine function is negative.
The reference angle for 120° is 180° - 120° = 60°.
The cosine of 120° is the same as the cosine of 60° in magnitude but negative because of the quadrant.
The value of cos 60° is 1/2, so cos 120 is -1/2.

Final Answer

The value of cos 120 is -½.

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