(後只人季 定理22:方阵A可逆的充要条件是AB0,并且 A 证明:必要性:已知A可逆,所以有AA1=E,因此 A4|=AA4HE=1≠0 故知4H=0 充分性: A=E 故A可逆,且A1= A定理2.2:方阵A可逆的充要条件是|A|≠0，并且 1 * | | A A A − = 1 1 | | | || | | | 1 0 AA A A E − − = = =  故知 |A|≠0. 充分性： * * | | | | A A A A E A A = = * 1 | | A A A − =