Saturday, 21 March 2015

calculus - Finding functions for the squeeze theorem for limxto0fracxsinx



I was solving a problem today and at one point I had to evaluate the lim. I know I could easily do this with L'hôspital's but I haven't learned that yet.



So what I did was try to use the squeeze theorem with the two bounding functions of f (x)=x^2 + 1 and h (x)=-x^2 +1 , and it worked (I think?) giving me the answer of 1. However, I was only able to choose those two functions after googling the graph of \lim_{x \to 0}{\frac {x}{\sin x}}, so that kind of ruins the purpose. Is there a way I could have visualized this or chosen other, more fitting functions?



Or maybe there is another way to solve this limit?



Answer



The hint given by OC-Sansoo is not enough, which only shows that \lim_{x\to 0^+}\frac{\sin x}{x}=0.



What you should do is to prove that \cos x < \dfrac{\sin x}{x} < 1 for x \in (-\pi/2,0).



Since x \in (-\pi/2,0), -x \in (0, \pi/2), hence, \cos (-x) < \dfrac{\sin (-x)}{-x} < 1, and hence \cos x < \dfrac{\sin x}{x} < 1.






Another way:




\lim_{x\to 0}\frac{\sin x}{x}=\lim_{x\to 0}\frac{\sin x-\sin 0}{x-0}=\sin'(x)\big|_{x=0}=\cos x|_{x=0}=\cos 0=1


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