原式= x?1 x+2 ? (x+2)(x?2) x(x?2) - x x?1 = x?1 x - x x?1 = (x?1)2?x2 x(x?1) = 1?2x x2?x ，当x= 1 2 时，原式= 1?2× 1 2 1 4 ? 1 2 =0．
