calculus - Limit definition of a derivative proof ...
math.stackexchange.com › questions › 3213974May 05, 2019 · From the definition of derivative, $$f'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}.$$ Since you are also told that $f$ satisfies the property $$f(x+y) = f(x)f(y)$$ for all $x, y$, it follows that $$f(x+h) = f(x)f(h),$$ hence $$f'(x) = \lim_{h \to 0} \frac{f(x)f(h) - f(x)}{h} = \lim_{h \to 0} f(x) \frac{f(h) - 1}{h} = f(x) \lim_{h \to 0} \frac{f(h) - 1}{h}.$$ The first step is simply applying the rule you were given.