Question

How do you evaluate `cos 2(pi/3)`?

Answer 1

`cos2(pi/3)=-1`

Explanation:

`cos 2(pi/3)=cos((2pi)/3)`

`(2pi)/3` is in quadrant Q II and has a reference angle of `pi/3`

`pi/3` is a standard angle with `cos(pi/3)=1/2`
but since `(2pi)/3` is in Q II
`cos((2pi)/3)=-cos(pi/3) =-1/2`