Theorem 20.6. Let A,B,C,and D be nonempty sets.Suppose that AB=CD= 0,A≈C,andB≈D.Then AUB≈CUD. Corollary 20.8. Let A and B be disjoint sets.If A and B are finite,then AUB is finite. Theorem 20.10. Let n be apositive integer.Then every subset of (1,2,3,...nis finite Corollary 20.11. Let S be a finite set.Then every subset of S is finite. Theorem 20.12. The union of two finite sets is finite