How do you find the reference angle for (25 pi)/6?

Express ##(25pi)/6## to degree measure then get the terminal side of the angle.
##(25pi)/6=(25cancelpi)/6(180^o/cancelpi)=750^o##
##750^o = 360^o + 360^o + 30^o## that means there are 2 rotation and remaining angle of ##30^o## lies on the ##I-Quadrant.## Therefore
##thetar = 30^o##
Reference angle ##thetar## by definition is the angle formed between the terminal side and the ##x-##axis.