Lim ((x+h)^3-x^3)/h при h>0
Lim ((x+h)^3-x^3)/h=lim ((x+h-x)*((x+h)^2+(x+h)x+x^2))/h=lim (h*((x+h)^2+(x+h)x+x^2))/h= lim (((x+h)^2+(x+h)x+x^2))=3x^2