x1+观 a,x+a,,=b 21 22 12 22 x1+ 11~1 12~2 a21x1+a222=b2 21   + = + = . , 21 1 22 2 2 11 1 12 2 1 a x a x b a x a x b , 2 22 1 12 1 b a b a D =    + = + = . , 21 1 22 2 2 11 1 12 2 1 a x a x b a x a x b . 21 2 11 1 2 a b a b D =