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I have tried to prove it by the definition of differentiability, but I have gotten nowhere. I am not very clear how to proceed. I would really appreciate your help.

janmarqz
  • 10,538

1 Answers1

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Notice that, since $f$ is linear:

$$ f(a+h) - f(a) = f(h) $$

hence the rest $R(h) = 0$, what gives the result.