Cosine is one of the most important functions of trigonometry which deals with the relationship between the angles and sides of a right angle triangle.

## Answer: The Value of cos 120° degree is -1/2.

Let us analyze trigonometric formulas step by step.

**Explanation: **

Let us write 120° as a sum of 90° and 30°.

Now we can apply the cos (a+b) formula: cos (a+b) = cos a cos b - sin a sin b

On substituting as 90° and b as 30°, we get,

cos 120° = cos 90° cos 30° - sin 90° sin 30°

cos 120° = 0 - 1/2

cos 120° = -1/2

### Thus, the value of cos 120° degrees is -1/2.