Friday, 18 May 2018

calculus - Should L'Hopital's Rule be used for this limit?



Question



$$\lim_{x \to 0} \left(\dfrac{\sin x}{x}\right)^{\dfrac{1}{1 - \cos x}}$$



Attempt




This limit is equal to,



$$\lim_{x \to 0} \exp\left({\dfrac{1}{1 - \cos x}\ln \left(\dfrac{\sin x}{x}\right)}\right).$$



I hope to solve this by focusing on the limit,



$$\lim_{x \to 0} {\dfrac{1}{1 - \cos x}\ln \left(\dfrac{\sin x}{x}\right)}.$$



Which is indeterminate of the form "$\frac{0}{0}$". However, repeated application of L'Hopital's Rule seems to lead to an endless spiral of trigonometric terms, but wolfram Alpha says the limit I am focusing on exists.




Is there another way to solve this?


Answer



It should be used. There may be some little mistakes in your computation.



$$\begin{align*}
\lim_{x\rightarrow0}\frac{1}{1-\cos x}\ln\frac{\sin x}{x}
&=\lim_{x\rightarrow0}\frac{\frac{\cos x}{\sin x}-\frac{1}{x}}{\sin x}\\
&=\lim_{x\rightarrow0}\frac{x\cos x-\sin x}{x\sin^{2}x}\\
&=\lim_{x\rightarrow0}\frac{-x\sin x}{\sin^{2}x+2x\sin x\cos x}\\

&=-\lim_{x\rightarrow0}\frac{1}{\frac{\sin x}{x}+2\cos x}\\
&=-\frac{1}{3}
\end{align*}$$


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