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 = \(-1\over 2\)

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