（x+1）^（1/3） - (x-1)^(1/3) = ((x+1)-(x-1))/((x+1)^(2/3)+(x+1)^(1/3)*(x-1)^(1/3)+(x-1)^(2/3))原式=lim(x->无穷)2*x^(2/3)/((x+1)^(2/3)+(x+1)^(1/3)*(x-1)^(1/3)+(x-1)^(2/3))2/3
