Hi!
Regarding your answers, friends if you study the book "Linear Systems And Signals" by B.P.Lathi, you will find that the author has said :
"By definition, a periodic signal x(t) remains unchanged when time-shifted by one period.For that reason a periodic signal must start at
t= - infinity: if it starts at some finite instant, say t=0, the time-shifted signal
x(t+T) would start at t= - T 0 ( - T naught) and x(t+ T0) would not be the same as x(t)."
Now I think that there is some solid reason because of which the author is using the word "must" .
Find that if you can and tell me again.I also try to find that reason