Tuesday, September 19, 2017

calculus - How to prove $lim limits_{x to 0}frac {tan (x)}{x} = 1$?



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



$$\lim \limits_{x \to 0}\dfrac {\sin (x)}{x} = 1
\qquad\text{and}\qquad\lim \limits_{x \to 0}\dfrac {\tan (x)}{x} = 1.$$



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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