2/3x*(-1/3)三分之二倍X的负三分之一次方！
lim [(x+dx)^(2/3)-x^(2/3)]/dx=lim[(x+dx)^2-x^2]/{[(x+dx)^(4/3)+x^(2/3)*(x+dx)^(2/3)+x^(4/3)]*dx}=lim(2x+dx)/[(x+dx)^(4/3)+x^(2/3)*(x+dx)^(2/3)+x^(4/3)]=2x/[3x^(4/3)]=2/3*x^(-1/3)
