Sunday, August 25, 2019

real analysis - How do i evaluate this limit :limxto0fracexx1x² without using Taylor expansion




I would like to evaluate this limit without using Taylor expansion:




lim .



Note: by Taylor Expansion i have got :\frac{1}{2} .



Thank u for any help .!!!!


Answer



METHODOLOGY 1: Use L'Hospital's Rule Successively



Repeated use of L'Hospital's Rule reveals




\begin{align} \lim_{x\to 0}\frac{e^x-x-1}{x^2}&=\lim_{x\to 0}\frac{e^x-1}{2x}\\\\ &=\lim_{x\to 0}\frac12 e^x=\frac12 \end{align}






METHODOLOGY 2: Integral representation of the numerator




Note that we can write the numerator as



\begin{align} e^x-x-1&=\int_0^x \int_0^t e^s \,ds\,dt\\\\ &=\int_0^x \int_s^x e^s\,dt\,ds\\\\ &=\int_0^x (x-s)e^s\,ds \end{align}



Now, we can use the Mean-Value-Theorem for integrals to reveal




\begin{align} e^x-x-1&=e^{s^*(x)}\int_0^x(x-s)\,ds\\\\ &=\frac12 x^2e^{s^*(x)} \end{align}



for some value of s^*(x) \in (0,x).



Finally, exploiting the continuity of the exponential function yield the coveted limit



\begin{align} \lim_{x\to 0}\frac{e^x-x-1}{x^2}&=\lim_{x\to 0}\frac{\frac12 x^2e^{s^*(x)}}{x^2}\\\\ &=\frac12 \end{align}



as expected!


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