Periodic function and substitution question

[cjl28]

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 question.
thanks for helping me!

 

[jbunniii]

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 question.
thanks for helping me!

Try breaking the integral on the left into two parts:

\int_{a}^{a+T} = \int_{a}^{T} + \int_{T}^{T+a}

and in the second part, use the fact that f is periodic.