Explanation:
Positive angles are measured counterclockwise from 0. Negative angles are measured clockwise from 0. So cos(-π/6) = √3/2.
Or, you can use angle reflection properties.
cos(-x) = cos x
cos(-π/6) = cos(π/6) = √3/2
Another way to look at it is to add 2π to the angle to turn it into a positive angle.
cos(-π/6) = cos(11π/6) = √3/2