11+a12 21x1+a2x2=b2 12 = 22 11 a 12~2 a21x1+a2)x2=b2 D 12 21 22   + = + = . , 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 22 11 12 a a a a D =