Question

Which of the following expressions could not be used to determine the exact value of cos 120°?

Answer 1

From the trigonometrical identities, we know that

`\cos 2x=\cos ^2x-\sin ^2x=2\cos ^2-1=1-2\sin ^2x`

These identites will give us cosine of 120 when x=60.

On the other hand,

`\sin 2x=2\sin x\cos x`

which will give us sine of 120 when x=60.

Therefore, by comparing with the given options, the answer is:

`2\sin 60\cos 60`