把三次根号下x看成t相当于(1-t^3)/(1-t)=1+t+t^2，值为3
1-x= [1-x^(1/3)].[1+x^(1/2) + x^(2/3)]lim(x->1) ( 1-x)/(1- x^(1/3))=lim(x->1) [1+x^(1/2) + x^(2/3)]=3
