Sunday, 30 June 2019

calculus - A limit without using L'hoptial's rule

Define the natural number $e$ by $e=\lim_{x\to 0} (1+x)^{1/x}$.



Then, I can prove $\lim_{x\to 0} \frac{e^x-1}{x}=1$.




Let $z=e^x-1$. Then, $x=\ln(z+1)$ and $$\lim_{x\to 0} \frac{e^x-1}{x}=\lim_{z\to 0} \frac{z}{\ln(z+1)}=\frac{1}{\ln e}=1\text{.}$$



Using a similar trick (without L'Hoptial's rule), can we prove $\lim_{x \to 0}\frac{e^x-1-x}{x^2}=\frac{1}{2}$?

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