limx→0arctanxe2x−1
How to do this without L'Hôpital and such? arctanx=y, then we rewrite it as limy→0ye2tany−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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