Saturday, 13 January 2018

calculus - Stuck on basic limit problem: limxto0fracsin(tanx)sinx



Consider lim. The answer is 1. This is clear intuitively since \tan x ≈ x for small x. How do you show this rigorously? In general, it does not hold that \lim_{x \to p} \frac{f(g(x))}{f(x)} = 1 if g(x) - x \to 0 as x \to p.



No advanced techniques like series or L'Hôpital. This is an exercise from a section of a textbook which only presumes basic limit laws and continuity of composite continuous functions.




This should be a simple problem but I seem to be stuck. I've tried various methods, including \epsilon-\delta, but I'm not getting anywhere. The composition, it seems to me, precludes algebraic simplification.


Answer



\lim_{x \to 0}\dfrac{\sin(\tan(x))}{\sin(x)}=\lim_{x \to 0}\dfrac{\sin(\tan(x))}{\sin(x)/x} \cdot \dfrac{1}{x} = \lim_{x \to 0}\dfrac{\sin(\tan(x))/\tan(x)}{\sin(x)/x} \cdot \dfrac{\tan(x)}{x}\text{.}
Now
\lim_{x \to 0}\sin(\tan(x))/\tan(x) = \lim_{\tan(x) \to 0}\sin(\tan(x))/\tan(x) = 1\text{,}
\lim_{x \to 0}\sin(x)/x = 1
and you can use this (or any of the other answers if you haven't covered derivatives) to show
\lim_{x \to 0}\tan(x)/x=\sec(0) = 1\text{.}


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