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

## Answer: The value of cos 120 degrees is -1/2.

Let us find the value of cos 120°with the help of the trigonometry formulas.

**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 'a' 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° is -1/2.