Let f: R-> R be a continuous function. Let T>0 be such that
f(x+T)= f(x) for all x.
We say that f is a periodic function with a period T>0.
Use an appropriate substitution to prove that for all real numbers a
\int^{a+T}_{a}f(x)dx = \int^{T}_{0}f(x)dx.
I have no idea how to do this...