Let $P = (\Omega, \mathcal{F}, \mathbb{P})$ be a D1159: Probability space such that

(i)	$X : \Omega \to \mathbb{R}$ is a D3161: Random real number on $P$
(ii)	\begin{equation} \mathbb{E} |X|^2 < \infty \end{equation}

Then \begin{equation} \mathbb{E}[(X - \mathbb{E} X)^2] = \mathbb{E} (X X) - \mathbb{E} X \mathbb{E} X \end{equation}