原式= (x?2)2 x(x?2) + x?2 x?1 = x?2 x - x?2 x?1 = (x?2)(x?1)?x(x?2) x(x?1) = 2?x x?1 ，当x= 1 2 时，原式= 2? 1 2 1 2 ?1 =-3．
