Let $[- a, a] \subseteq \mathbb{R}$ be a D544: Closed real interval such that

(i)	$f : [- a, a] \to \mathbb{R}$ is a D1760: Riemann integrable real function on $[- a, a]$
(ii)	$f$ is an D5480: Even real function

Then \begin{equation} \int^a_{-a} f(x) \, d x = 2 \int^a_0 f(x) \, d x \end{equation}