How do you determine whether a function is periodic or not?

[Harouls]

Specifically on a calculator which makes it hard to determine the period of the function.
I realize you're supposed to find the period, and add random X values to that period and see if the Y is the same for both the X + period and just the X, but I have to go over 10 functions and determine whether they're periodic or not, and I get confused as to whether the function might change in certain areas but not others.
A descriptive answer would be appreciated, thanks.

 

[vivekrai]

Have a quick look here (with patience) :

http://cnx.org/content/m15292/latest/