Question Number 134809 by liberty last updated on 07/Mar/21
$${Show}\:{if}\:{f}\:''\left({a}\right)\:{exist}\:{then}\: \\ $$$${f}\:''\left({a}\right)=\underset{{h}\rightarrow\mathrm{0}} {\mathrm{lim}}\:\frac{{f}\left({a}+{h}\right)+{f}\left({a}−{h}\right)−\mathrm{2}{f}\left({a}\right)}{{h}^{\mathrm{2}} } \\ $$