I为单位矩阵
(A-I)(B-I)
=A(B-I)-I(B-I)
=AB-A-B+I
=I
因此,(A-I)和(B-I)互为逆矩阵
因此
(B-I)(A-I)=I
即BA-A-B+I=I
BA=A+B=AB