Let $X \in \text{Random}(\mathbb{R})$ be a D3161: Random real number such that
(i) | \begin{equation} \mathbb{E} |X|^2 < \infty \end{equation} |
Then
\begin{equation}
\mathbb{E}[(X - \mathbb{E} X)^2]
= \mathbb{E}(X^2) - (\mathbb{E} X)^2
\end{equation}