Theorem 20.6. Let A,B,C,and D be nonempty sets.Suppose that AB=CD= g,A≈C,amdB≈D.Then AUB≈CUD Corollary 20.8. Let A and B be disjoint sets.If A and B are finite,then AUB is finite. Exercise 21.12.Use induction to prove the following.Let m Z.If A1,A2,...,Am are finite sets,then the union Ajis finite. Let S be a finite set.Then every subset of S is finite. Theorem 20.12. The union of two finite sets is finite