因为（x的平方-2x+1）/（x的平方-1）÷（x-1）/（x的平方+x）-x的值与x无关：
（x的平方-2x+1）/（x的平方-1）÷（x-1）/（x的平方+x）-x
=(x-1)^2/[(x+1)(x-1)] * x(x+1)/(x-1) -x
=x -x
=0