July 5th, 2019, 02:23 PM  #1 
Senior Member Joined: Oct 2015 From: Greece Posts: 137 Thanks: 8  I can't show why x>0 of sin(x)/x is 1!!!
I started using $\displaystyle x \leq \sin x \leq x$ then $\displaystyle \frac{x}{x} \leq \frac{\sin x}{x} \leq \frac{x}{x}$ But there is not a single value L for $\displaystyle \lim_{x \to 0} \frac{x}{x}=L$ . Well I have a theory but I'm not sure if it stands. $\displaystyle \lim_{x \to 0^}\frac{x}{x} = (1) = 1 $ $\displaystyle \lim_{x \to 0^+}\frac{x}{x} = 1 $ So $\displaystyle \lim_{x \to 0} \frac{\sin x}{x}=1$ But can I actually do that? I took different kind of limits in both cases. This seems like I'm breaking the inequality since I'm taking different limits at once! It feels like I'm doing something like this: $\displaystyle x + 5 = 5 <=> (x + 5) \cdot 10 = 5 \cdot 20$ I multiplied the two sides of the equation with not the same number. Last edited by skipjack; July 5th, 2019 at 09:50 PM. 
July 5th, 2019, 02:46 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,531 Thanks: 1390  
July 5th, 2019, 06:07 PM  #3 
Member Joined: Oct 2018 From: USA Posts: 88 Thanks: 61 Math Focus: Algebraic Geometry 
The Calculus way to go about this would be L'hopitals rule: If $\lim_{x \to a} \frac{f(x)}{g(x)}$ is indeterminate ($\frac{0}{0}$, $\frac{\infty}{\infty}$), then $\displaystyle \lim_{x \to a}\frac{f(x)}{g(x)} = \lim_{x \to a}\frac{f^{\prime}(x)}{g^{\prime}(x)}$ So, since $\lim_{x \to 0} \frac{\sin{x}}{x} = \frac{0}{0}$ $\displaystyle \lim_{x \to 0} \frac{\sin{x}}{x} = \lim_{x \to 0}\frac{\cos{x}}{1} = \cos{0}=1$ But the method Romsek sent is a much more concrete way to go about it. 
July 5th, 2019, 10:03 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,941 Thanks: 2210  You assume that the derivative of sin(x) is cos(x), but the usual proof of that assumes knowledge of the limit you're trying to find.
Last edited by skipjack; July 5th, 2019 at 11:44 PM. 
July 5th, 2019, 10:37 PM  #5 
Member Joined: Oct 2018 From: USA Posts: 88 Thanks: 61 Math Focus: Algebraic Geometry 
That's absolutely true, which is why I mentioned the other method is much more concrete. But generally, if I were tossed this problem without having to build from the ground up I would use L'hôpital. Last edited by skipjack; July 5th, 2019 at 11:43 PM. 
July 6th, 2019, 12:24 AM  #6  
Senior Member Joined: Oct 2015 From: Greece Posts: 137 Thanks: 8  Quote:
Also my original post of taking the left handed side and the right handed side limit is wrong write? −limx→0−xx=−(−1)=1 limx→0+xx=1 So limx→0sinxx=1  
July 6th, 2019, 06:34 AM  #7 
Member Joined: Oct 2018 From: USA Posts: 88 Thanks: 61 Math Focus: Algebraic Geometry 
When you took $\lim_{x \to 0^{}} \frac{x}{x}$ , the added negative sign breaks it since the fraction is already negative on the left. This creates the squeeze $\displaystyle \frac{x}{x} \leq \frac{\sin{x}}{x} \leq \frac{x}{x}$ Which would imply $\frac{x}{x} = \sin{x}$ , which isn't always true. 
July 6th, 2019, 01:53 PM  #8 
Senior Member Joined: Sep 2016 From: USA Posts: 635 Thanks: 401 Math Focus: Dynamical systems, analytic function theory, numerics 
So much is wrong in this post which isn't really worth trying to correct. Two things are bad enough they should be pointed out. 1. You absolutely can NOT use L'Hospital's rule to "show" that $\lim\limits_{x \to 0} \frac{\sin x}{x} = 1$. At best this is circular logic, but I would say it's even worse than that. It is a complete misunderstanding of what L'Hospital's rule means. 2. Romsek's link is technically accurate, but I don't think it's helpful. It begins with 2 inequalities which "can be shown". But given these 2 inequalities it's completely trivial to compute this limit. The important part (which also has all of the insight here) is showing those two inequalities. This is typically done by a geometric argument which is difficult to carry out without drawing the right picture. Last edited by skipjack; July 7th, 2019 at 01:06 AM. 
July 6th, 2019, 02:09 PM  #9  
Senior Member Joined: Sep 2015 From: USA Posts: 2,531 Thanks: 1390  Quote:
I don't suppose it would be too much to ask that you post what you consider an adequate solution to the problem being asked would it?  
July 8th, 2019, 05:29 AM  #10  
Senior Member Joined: Oct 2015 From: Greece Posts: 137 Thanks: 8  Quote:
 

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