Thursday, 2 November 2017

algebra precalculus - Finding the limit without L'Hospital's rule.



Find the value of n for the given limit.




\lim\limits_{x \to 1}\frac{π/4-\tan^{-1}x}{e^{\sin(\ln x)}-x^n} =\frac 18.



ATTEMPT:



I tried expanding the denominator by Maclaurin's series, but the term e^{\sin(\ln x)} has a very complicated expansion. Is there any other way to solve this without L'Hospital's rule?


Answer



For x=1+h, we have \ln x=h-\frac12 h^2+O(h^3),
so \sin\ln x=h-\frac12 h^2+O(h^3)
and \exp\sin\ln x = 1+(h-\frac12 h^2)+\frac12(h-\frac12 h^2)^2+O(h^3)=1+h+O(h^3)

and finally in the denominator
\exp\sin\ln x-x^n=(1-n)h-{n\choose2}h^2+O(h^3).
In the numerator
\arctan(1+h)=\frac\pi4+\frac12 h+O(h^2).
Thus for n\ge2 we have
\frac{ \frac\pi4-\arctan x}{\exp\sin\ln x-x^n}=\frac{-\frac12h+O(h^2)}{(1-n)h+O(h^2)}=\frac1{2(n-1)}+O(h)
(and for n=1 there is divergence). Apparently we want n=5. (And apparently O(h^3) in the denominator was overkill)


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