Cardinality of Set Union/Corollary

Theorem
Let $S_1, S_2, \ldots, S_n$ be finite sets which are pairwise disjoint.

Then:
 * $\ds \card {\bigcup_{i \mathop = 1}^n S_i} = \sum_{i \mathop = 1}^n \card {S_i}$

Specifically:
 * $\card {S_1 \cup S_2} = \card {S_1} + \card {S_2}$

Proof
As $S_1, S_2, \ldots, S_n$ are pairwise disjoint, their intersections are all empty.

The Cardinality of Set Union holds, but from Cardinality of Empty Set, all the terms apart from the first vanish.