●分块求逆(重点) 6没A= 0 若A1≠(=1,2,…,s,则4≠0,并有 0若 Ai  0(i = 1,2,  ,s),则 A  0,并有 . 2 1               = As A A A  o o (6) , 2 1               = As A A A  设 o o −1 −1 −1 −1 分块求逆（重点）