当A≠0时, 1①12 a1A1+a12412+…+an4n=4 anAn+a,A,+…+anAn=A n nn n O当 A  0时,               n n nn n n n n nn n n A A A A A A A A A a a a a a a a a a AA               1 2 12 22 2 11 21 1 1 2 21 22 2 11 12 1 a11A11  a12A12  a1nA1n  A an1An1  an2An2  annAnn  A ,        A A A A O O 