What is the Value of Cos 120 Degrees ?

Solution :

The value of cos 120 degrees is $$-1\over 2$$.

Proof :

We Know that sin 60 = $$\sqrt{3}\over 2$$ and cos 60 = $$1\over 2$$

By using cos 2A formula,

cos 120 = $$cos^2 60$$ – $$sin^2 60$$ = $$1\over 4$$ – $$3\over 4$$

$$\implies$$  cos 120 = $$-1\over 2$$