f(0)=0
g(x)
=f(x)/x ; x≠0
=a ; x=0
lim(x->0) g(x)
=lim(x->0) f(x)/x (0/0)
=lim(x->0) f'(x)
=f'(0)
x=0, g(x) 连续
=> a=f'(0)
(2)
g(x)
=f(x)/x ; x≠0
=f'(0) ; x=0
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