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题:$f\in\mathscr{C}((a,b)),\,c\in(a,b),\;\displaystyle\lim_{x\to c}f'(x)$存在,$\,f'(c)$是否存在?bof 解:由中值定理,$\displaystyle\lim_{x\to c}{\small\frac{f(x)-f(c)}{x-c}}=\lim_{h\to 0}f'(c{\small+\,\xi_h})=\lim_{x\to c}f'(x)$`dA_3 $\therefore\quad f'(c)=f'(c\pm 0)\,$存在.$\small\quad\square$\~Z
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