(X+h)^3 + (x+h)-(x^3+x)
\(\dfrac{(x+h)^3 + (x+h)-(x^3+x)}{h}=\\ \dfrac{x^3+3x^2h+3xh^2+h^3+x+h-x^3-x}{h}=\\ \dfrac{3x^2h+3xh^2+h^3+h}{h}=\\3x^2+3xh+h^2+1\)
