Let $p$ be a real number and let $f: \mathbb{R} \rightarrow \mathbb{R}$ be a continuous function such that $f(x+p)=f(x)$ for all $x \in \mathbb{R}$. (Such a function is said to be periodic.) Show that the integral $\int_{a}^{a+p} f(t) d t$ has the same value for every real number $a$. (Hint: Part (ii) of Proposition 6.21.)