Question

Why is Cos120=-cos60?

Answer 1

`cos 120 = cos(180-60) = cos 180 cos 60 + sin 180 sin 60`

(using the formula, `cos(a+b)=cos a cos b+sin a sin b`

Now,we know, `cos 180=-1,cos 60=1/2,sin 180=0,sin 60=(sqrt(3))/2`

So,putting the value,we get,

`cos 120=-1×(1/2) + 0×(sqrt(3))/2=-1/2=-cos 60`

Remember the formula `cos(180-theta)=-cos theta` for shorter approach.