Tuesday, 16 June 2015

Computing limit of fracxsinxxtanx without L'Hôpital




I want to compute following:




lim



I have tried to calculate this with l’Hôpital's rule. l’Hôpital's rule states that:
\lim_{x\rightarrow a} \frac{f(x)}{g(x)}=\lim_{x \rightarrow a } \frac{f'(x)}{g'(x)}



Now i can get to the right result with l’Hôpital's rule but it took a little over two pages on paper. Had to use l’Hôpital's rule 4 times. How do you solve this without l’Hôpital's rule ?



The result appears to be(with l’Hôpital's rule):



\lim_{x \rightarrow 0} \frac{x-\sin(x)}{x-\tan(x)}=-\frac{1}{2}




If someone can provide alternative solution to this problem that would be highly appreciated.


Answer



Utilizing the Taylor expansions



\sin(x)=x-\frac{1}{6}x^3+\mathcal{O}(x^5)



\tan(x)=x+\frac{1}{3}x^3+\mathcal{O}(x^5), we get



\frac{x-\sin(x)}{x-\tan(x)}=\frac{x-(x-\frac{1}{6}x^3+\mathcal{O}(x^5))}{x-(x+\frac{1}{3}x^3+\mathcal{O}(x^5))}=\frac{\frac{1}{6}x^3+\mathcal{O}(x^5)}{-\frac{1}{3}x^3+\mathcal{O}(x^5)}\to-\frac{1}{2} as x\to0.



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