Countable additivity
= Countable additivity
For pairwise disjoint measurable sets $E_n$, countable additivity means
$$
\mu\!\left(\bigcup_{n=1}^{\infty}E_n\right)
=\sum_{n=1}^{\infty}\mu(E_n).
$$
= Countable additivity
For pairwise disjoint measurable sets $E_n$, countable additivity means
$$
\mu\!\left(\bigcup_{n=1}^{\infty}E_n\right)
=\sum_{n=1}^{\infty}\mu(E_n).
$$