y=1/x是基本初等函数（1+x^2）/(1-x^2)的导数等于4x/(1-x^2)^2
导数=[(1+x^2)'(1-x^2)-(1+x^2)(1-x^2)']/(1-x^2)^2 =[2x*(1-x^2)+2x*(1+x^2)]/(1-x^2)^2=4x/(1-x^2)^2