Let $M = (X, \mathcal{F}, \mu)$ be a D1158: Measure space.
Let $E, F \in \mathcal{F}$ each be a D1109: Measurable set in $M$ such that

(i)	\begin{equation} \mu(E), \mu(F) < \infty \end{equation}

Then \begin{equation} \mu(E \cup F) = \mu(E) + \mu(F) - \mu(E \cap F) \end{equation}