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[1CV]Let \(I=(a,b)ββ\) be an open interval. Let \(f:Iββ\) be differentiable: show that \(f'\) is continuous, if and only if for every \(x\)
\[ f'(x) = \lim _{(s,t)β (x,x), sβ t} \frac{f(t)-f(s)}{t-s} ~ . \]
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