I know what ever you replied.What I am asking is that why don't we take another domain which contents all the values.Then the relations are going to change?please please please please kindly reply

I think you want to ask that why cant we take any other values than the principles values as domain. Actually we can take any other values in domain and it will not at all change their properties. But the fact is that the values are different from 0 to 360 degree. Then they repeats periodically(361 to 720 will also have the very same values).And we does not take any other values as you know angle starts from 0. SO We take 0 t0 360 as principle values as they are the first to come and there is a rule of first come , first serve!!!!

Approved