Number of Disjoint Unions
Result
We call two sets $A$, $B$ disjoint or non-overlapping1 if $A \cap B = \varnothing$.
Suppose $A$ and $B$ are disjoint finite sets.
Then $\num{A \cup B} = \num{A} + \num{B}$
This is often called the
addition principle or the
rule of sum.- Future sheets may move this definition elsewhere. ↩︎