a21x1+a2x2=b2 D I1 21X1+a2 D   + = + = . , 2 1 1 2 2 2 2 1 1 1 1 2 2 1 a x a x b a x a x b , 2 22 1 12 1 b a b a D =    + = + = . , 2 1 1 2 2 2 2 1 1 1 1 2 2 1 a x a x b a x a x b , 21 22 11 12 a a a a D =