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
1+x≤ex≤11−x
for x<1.
And in THIS ANSWER, I showed using only elementary inequalities from geometry that the arctangent function satisfies the inequalities
|x|√1+x2≤|arctan(x)|≤|x|
for all x.
Using (1) and (2) we can write for 1>x>0
x√1+x2(2x1−2x)≤arctan(x)e2x−1≤x2x
whereupon applying the squeeze theorem to (3), we find that
limx→0+arctan(x)e2x−1=12
Similarly, using (1) and (2) for x<0 we can write
x(2x1−2x)≤arctan(x)e2x−1≤x√1+x2(2x)
whereupon applying the squeeze theorem to (4), we find that
limx→0−arctan(x)e2x−1=12
Inasmuch as the limits from the right and left sides are equal we can conclude that
limx→0arctan(x)e2x−1=12
No comments:
Post a Comment