#arctan(−1)=(npi-pi/4)#, where #n# is an integer i.e. #arctan(−1)# can take values such as #{.......,-9pi/4,-5pi/4,-pi/4,3pi/4,7pi/4,.....}#
Explanation:
#arctan(−1)# is the angle whose tangent is #-1#.
As #tan(-pi/4)=-tan(pi/4)=-1# and tangent of angle has a cycle of #p# radians
We can generally put #arctan(−1)=(npi-pi/4)#, where #n# is an integer i.e. #arctan(−1)# can take values such as #{.......,-9pi/4,-5pi/4,-pi/4,3pi/4,7pi/4,.....}#