\begin{definition}[Definition:Quadratic Mean]
Let $x_1, x_2, \ldots, x_n \in \R$ be real numbers.
The '''quadratic mean''' of $x_1, x_2, \ldots, x_n$ is defined as:
:$Q_n := \ds \sqrt {\frac 1 n \sum_{k \mathop = 1}^n x_k^2}$
\end{definition}