Monday, 10 February 2014

calculus - How to prove limlimitsxto0fractan(x)x=1?



I was going through a Calculus Textbook, when came across the following two identities :



lim



The first one is really popular/famous and has many proofs. But I am more interested in the second one.




The fraction \frac {\tan (x)}{x} becomes \frac {0}{0} at x=0, so the easiest approach is the L'Hopital's Rule.



So \lim \limits_{x \to 0}\dfrac {\tan (x)}{x} = \dfrac{\frac{d}{dx}\tan(x)}{\frac{d}{dx}x} = \dfrac{\sec^2(x)}{1} \\ \implies \lim \limits_{x \to 0}\dfrac {\tan (x)}{x} = \sec^2(0) = 1.



A geometrical proof of the same can be found here. I am looking for some non-geometrical proof for this identity.


Answer



\lim \limits_{x\rightarrow 0} \dfrac{\tan{x}}{x}=\lim \limits_{x\rightarrow 0} \dfrac{\sin{x}}{x}\lim \limits_{x\rightarrow 0} \frac{1}{\cos{x}}=1\times 1=1




since the two limits exist.


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