Question: is cos x a periodic function?
Answer: If we look at the cosine function from x = 0 to x = 2π, we have an interval of the graph that's repeated over and over again in both directions, so we can see why the cosine function is a periodic function. This interval from x = 0 to x = 2π of the graph of f(x) = cos(x) is called the period of the function.