g'(0)=lim(h->0) [ f(h)/h -f'(0) ]/h (0/0)=lim(h->0) [ hf'(h)- f(h) ]/h^2 (0/0)=lim(h->0) [ hf''(h) +f'(h)- f'(h) ]/(2h)=lim(h->0) f''(h)/2=f''(0)/2
