f(x)=g(x)/x,x!=0;f(x)=o,x=0
g(x)在x=0的Taylor展开式为个g(x)=g(0)+g'(0)x+g"(0)x^2/2!+O(x^2)
f'(0)=lim(x->0)[(f(x)-f(0))/(x-0)]
=lim(x->0)f(x)/x
=lim(x->0)g(x)/x^2
=lim(x->0)[g(0)+g'(0)x+g"(0)x^2/2!+O(x^2)] /x^2
代入 g(0)=g'(0)=0 ,g"(0)=2,
=1