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}
\text{Var} X
= \mathbb{E}(X X) - \mathbb{E} X \mathbb{E} X
\end{equation}