coterminal angles are obtained by adding some multiple of 360 degrees or 2pi radians to the original angle.

a) 8pi/4

a positive coterminal angle is: 8pi/4 + 2pi = 2pi + 2pi = 4pi

a negative coterminal angle is: 8pi/4 - 2*2pi = 2pi - 4pi = -2pi

b) 8pi/45

a positive coterminal angle is: 8pi/45 + 2pi = 98pi/45

a negative coterminal angle is: 8pi/45 - 2pi = -82pi/45

see Find Coterminal Angles for more info