[6(x-y)/x]/[x-(2xy-y^2)/x]=[6(x-y)/x]*[x/(x^2-2xy+y^2)]=6(x-y)/(x-y)^2=6/(x-y)
代入：x=2^10,y=10^3
则：原式＝6/(2^10-10^3)=6/24=1/4