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 = \(-1\over 2\)