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.