\begin{definition}[Definition:Arithmetic Mean]
Let $x_1, x_2, \ldots, x_n \in \R$ be real numbers.
The '''arithmetic mean''' of $x_1, x_2, \ldots, x_n$ is defined as:
:$\ds A_n := \dfrac 1 n \sum_{k \mathop = 1}^n x_k$
That is, to find out the '''arithmetic mean''' of a set of numbers, add them all up and divide by how many there are.
\end{definition}