Sunday, December 11, 2016

Limit involving exponentials and arctangent without L'Hôpital



limx0arctanxe2x1


How to do this without L'Hôpital and such? arctanx=y, then we rewrite it as limy0ye2tany1, 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+xex11x



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



x1+x2(2x12x)arctan(x)e2x1x2x



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



limx0+arctan(x)e2x1=12



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




x(2x12x)arctan(x)e2x1x1+x2(2x)



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



limx0arctan(x)e2x1=12




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



limx0arctan(x)e2x1=12




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