0 )设 A= 王若4≠0=12.,则A≠0并有 工工工 NM…A 0 1 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