充分性：由于 a 2 AA= 422 0 n2 .o A -AB充分性: 由于 =  AA                             n n n n n n n n n n n n A A A A A A A A A a a a a a a a a a               1 2 1 2 2 2 2 1 1 2 1 1 1 2 2 1 2 2 2 1 1 1 2 1               = A A A        0 0 0 0 0 0 = A E