11+a12x a21x1+a2x2=b2 D1 12 = 22 I1 11 +12x2 211+a 22 D,=ll 21 上   + = + = . , 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 =