4.2 Permutations of sets 842.1 Basic counting principles Theorem 4.3(Addition principle): IfA, A2, .. A are disjoint sets, then the number of elements in the union of these sets is the sum of the numbers of elements in them 令|A1UA20…∪An=A1+A2+.+n4.2 Permutations of sets ❖4.2.1 Basic counting principles ❖ Theorem 4.3(Addition principle): If A1 , A2 , … , An are disjoint sets, then the number of elements in the union of these sets is the sum of the numbers of elements in them. ❖ | A1∪A2∪…∪An |=|A1 |+|A2 |+…+|An |