I found this problem in a problem book. I tried to use mean value theorem twice. $ $ \lim_{h\to0^{+}}4h^{-1}(\frac{f(x+h)-f(x)}{h}-\frac{f(x)-f(x-h)}{h})$ $ but $ f(x)$ may not even have a derivative. Would you help me find it out? Best regards.Read more