xy²/(x²y-y) × x²/(x²+x)=xy²/[(x+1)(x-1)y]×x²/[x(x+1)]=x²y/[(x+1)²(x-1)]
(x²-3x)/(x²-5x) × 2x-10/(x²-6x+9)=x(x-3)/[x(x-5)]× 2(x-5)/(x-3)²=2/(x-3)
x²-1/(x²-x-2)=(x+1)(x-1)/[(x-2)(x+1)]=(x-1)/(x-2),将x＝1/2代入,得（1/2-1）/（1/2-2）＝1/3
[x-x/(x+1)]/[x/(2x-4)] ＝[x(x+1)-x]/(x+1)× [2(x-2)/x]=2x(x-2),将x=√2+1代入得
2*（√2+1）（√2+1-2）＝2*（√2+1）（√2-1）＝2