f(x)
=g(x)/x ; x≠0
=0 ; x=0
g(0)=g'(0) =0 g''(0)=3
lim(x->0) g(x)
= lim(x->0) g(x)/x
=g'(0)
=0
x=0 , f(x) 连续
f'(0)
= lim(h->0) [ g(h)/h - f(0) ] /h
= lim(h->0) g(h)/h^2 (0/0 分子分母分别求导)
= lim(h->0) g'(h)/(2h) (0/0 分子分母分别求导)
= lim(h->0) g''(h)/2
=g''(0)/2
=3/2
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