TITLE: How to determine the periods of a periodic function?
QUESTION [5 upvotes]: I am aware of the other similar questions but was not able to figure out what I want to know from those question thus posting it here.
Given a periodic function $f(x)=sin(x)$,

Why is the period of above function becomes $2\pi$?
for function $sin(nx)$ , I am being told to use this formula $2\pi/n$ to get the period. But I am not able to fully understand the concept behind it.
I am using sin function for the sake of simplicity. My question is how do I quickly work out what are the periods of a certain periodic function(preferably without drawing out the graph)?
for example, $cos2t$ has periods $\pi,2\pi,3\pi,...$ (I have no idea why)
Also, there are many different types of periodic waves apart from sine and cosine. So, what would be the best way to get their periods if the only thing I am being given is the formula of the function.
For your info, I am supposed to find the smallest period so that I can work out the Fourier series of that function.

REPLY [2 votes]: You just need to solve the equation $$f(x) = f(x + T)$$ for $T$ using all available means suitable for solving the equation you get.