=\lim_{h=(h_1,h_2,h_3) \rightarrow 0}\frac{h_1(h_2+y_0)h_3^2-h_1^3+h_1(h_1^2+h_3^2)}{\sqrt{h_1^2+h_2^2+h_3^2}\cdot (h_1^2+h_3^2)}=