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 = $-1\over 2$