Tuesday, January 7, 2020

calculus - Finding limxto+infty(fracx+lnxxlnx)fracxlnx



Find lim. I tried using l'Hospital rule with the continuity of e function. Also tried using Taylor expansion with no success. What should I do? Thank you.



Answer



\begin{align*} \lim_{x\to+\infty}\left(\frac{x+\log x}{x-\log x}\right)^{\frac x{\log x}} =&\lim_{x\to+\infty}\left(1+\frac{2\log x}{x-\log x}\right)^{\frac x{\log x}}\\ =&\lim_{x\to+\infty}\left(1+\frac{2}{\frac{x}{\log x}-1}\right)^{\frac x{\log x}}\\ =&\lim_{x\to+\infty}\left(1+\frac{2}{\frac{x}{\log x}-1}\right)^{\frac x{\log x}-1}\left(1+\frac{2}{\frac{x}{\log x}-1}\right)\\ =&e^2 \end{align*}


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