将分子和分母同除以 y² ,可得：(x²-2xy+y²)/(x²-xy+y²)= [(x/y)²-2(x/y)+1]/[(x/y)²-(x/y)+1]= [(1/2)²-2(1/2)+1]/[(1/2)²-(1/2)+1]= 1/3（即：3分之1）...