1.约分1/(x-y)^2=1/x^2-2xy+y^2
2.通分x^2+x/x^2-1=x+x^2(x^2-1)/x^2-1=x(x^2-1+1)/x^2-1=x^3/x^2-1
3.平方差公式(x-3)(x+3)/(x-3)^2=x+3/x-3