given that f(x) is a differentiable function.
$$lim_{\delta \to 0} \big(\frac{f(x+\delta x)}{f(x)}\big)^{\frac{1}{\delta}} ---------(1)$$
by direct substitution it is in the form of $1^\infty$.
so we can use this formula
$$lim_{\delta \to 0} \big(1+g(\delta)\big)^{h(\delta)}=e^{lim_{\delta \to 0} g(\delta).h(\delta)$$
here , from(1) we get
$$\displaystyle \Rightarrow lim_{\delta \to 0} \big(1+\frac{f(x+\delta x)}{f(x)} -1\big)^{\frac{1}{\delta}}$$
$$\displaystyle \Rightarrow lim_{\delta \to 0} \big(1+\frac{f(x+\delta x)-f(x)}{f(x)}\big)^{\frac{1}{\delta}}$$
here, $g(\delta)=\frac{f(x+\delta x)-f(x)}{f(x)}$ and $h(\delta )= \frac{1}{\delta}$
$$\displaystyle \Rightarrow e^{lim_{\delta \to 0} g(\delta).h(\delta)$$
$$\displaystyle \Rightarrow e^{lim_{\delta \to 0}\frac{f(x+\delta x)-f(x)}{f(x)} .\frac{1}{\delta}$$
by rearranging terms
$$\Rightarrow e^{lim_{\delta \to 0}\frac{f(x+\delta x)-f(x)}{\delta} .\frac{1}{f(x)}$$
by replacing the Definition of derivative $lim_{\delta \to 0}\frac{f(x+\delta x)-f(x)}{\delta} =f'(x)$. we get
$$\Rightarrow e^{\frac{f'(x)}{f(x)}}$$