# Inclusion-Exclusion Principle **A method of calculating the [[cardinality]] of the [[union]] of two finite sets.** > [!Example] Inclusion-Exclusion Principle > For two finite sets $A$ and $B$, > $|A\cup B|=|A|+|B|-|A\cap B|$ > > For three finite sets $A$, $B$, and $C$, > $|A\cup B\cup C|=|A|+|B|+|C|-|A\cap B|-|A\cap C|-|B\cap C|+|A\cap B\cap C|$