Question

How do you find the reference angle for `(-5pi`/6#?

Answer 1

http://www.mathopenref.com/reference-angle.html
Definition: The smallest angle that the terminal side of a given angle makes with the x-axis.

Assuming a base side for the angle along the X-axis, the reference angle is the smallest acute angle as measured from the X-axis

`(- (5pi)/6)` is `(-pi/6)` short of being a straight line

So the reference angle for `(- (5pi)/6)` is
`pi/6`