(B.)lim_(h to 0)(f(a+2h)-f(a+h))/(h)存在. (C.)lim_(h to 0)(f(a+h)-f(a-h))/(2h)存在. (D.)lim_(h to 0)(f(a)-f(a-h))/(h)存在.

【例20】(1989,数三)设f(x)在x=a的某个邻域内有定义,则f(x)在x=a处可导的一个充分条件是 (
A.)$\lim_{h \to 0}h\left[f\left(a+\frac{1}{h}\right)-f(a)\right]$存在. (
B.)$\lim_{h \to 0}\frac{f(a+2h)-f(a+h)}{h}$存在. (
C.)$\lim_{h \to 0}\frac{f(a+h)-f(a-h)}{2h}$存在. (
D.)$\lim_{h \to 0}\frac{f(a)-f(a-h)}{h}$存在.

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