分式计算 1-x/x²+x-2÷x²-1/x+2

（1-x）/（x²+x-2）÷[(x²-1)/(x+2)]
=-(x-1)/[(x-1)(x+2)]÷[(x-1)(x+1)/(x+2)]
=-1/(x+2)×(x+2)/[(x-1)(x+1)]
=1/(1-x²)