Friday, December 29, 2017

Limit involving exponentials and arctangent without L'Hôpital



lim
How to do this without L'Hôpital and such? \arctan x=y, then we rewrite it as \lim_{y\to0}\frac y{e^{2\tan y}-1}, but from here I'm stuck.


Answer



I thought it might be instructive to present a way forward that goes back to "basics." Herein, we rely only on elementary inequalities and the squeeze theorem. To that end, we proceed with a primer.





PRIMER ON A SET OF ELEMENTARY INEQUALITIES:



In THIS ANSWER, I showed using only the limit definition of the exponential function and Bernoulli's Inequality that the exponential function satisfies the inequalities



\bbox[5px,border:2px solid #C0A000]{1+x\le e^x\le \frac{1}{1-x}} \tag 1



for x<1.



And in THIS ANSWER, I showed using only elementary inequalities from geometry that the arctangent function satisfies the inequalities




\bbox[5px,border:2px solid #C0A000]{\frac{|x|}{\sqrt{1+x^2}}\le |\arctan(x)|\le |x|} \tag 2



for all x.







Using (1) and (2) we can write for 1>x>0



\frac{x}{\sqrt{1+x^2}\left(\frac{2x}{1-2x}\right)}\le \frac{\arctan(x)}{e^{2x}-1}\le \frac{x}{2x} \tag 3




whereupon applying the squeeze theorem to (3), we find that



\lim_{x\to 0^+}\frac{\arctan(x)}{e^{2x}-1}=\frac12



Similarly, using (1) and (2) for x<0 we can write



\frac{x}{\left(\frac{2x}{1-2x}\right)}\le \frac{\arctan(x)}{e^{2x}-1}\le \frac{x}{\sqrt{1+x^2}\,\left(2x\right)} \tag 4



whereupon applying the squeeze theorem to (4), we find that




\lim_{x\to 0^-}\frac{\arctan(x)}{e^{2x}-1}=\frac12




Inasmuch as the limits from the right and left sides are equal we can conclude that



\bbox[5px,border:2px solid #C0A000]{\lim_{x\to 0}\frac{\arctan(x)}{e^{2x}-1}=\frac12}



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