推论：若AB=E(或BA=E), 则B=A 证明：AB=E=1.故A≠0 因而A存在，于是 B=EB=(AAB =A(AB=AE =A推论：若 AB = E (或 BA = E）， . −1 则B = A 证明： A  B = E =1. A  0, −1 A B EB (A A)B −1 = = A AB A E 1 1 ( ) − − = = 故 因而 存在，于是 1 A − =