Lim x->0 (sin x / x) =1 contradiction?

  • #1
lim x->0 (sin x / x) =1......contradiction?

sin(x)/x =1 (limit x to 0)
this is an identity proved by using geometry and squeeze theorem ...right?

now today i came across another question and doing it my way ....gives me two answers;)
the question is limit x-->0 of [ x*sin(1/x)]

my first approach was using the above identity by rewriting the question as follows:
sin(1/x)/(1/x)....it means the same thing and is now in the form so that we can use the identity....so limiting x->0 must give us 1 according to the identity

now i thought to use the squeeze theorem as shown below:
-1≤sin(1/x)≤1
-x≤x*sin(1/x)≤x
now x goes to 0 so:
0≤x*sin(1/x)≤0
so x*sin(1/x)=0 as x goes to 0

now which method which is correct ?
 

Answers and Replies

  • #2
disregardthat
Science Advisor
1,861
34


sin(1/x)/(1/x) as x goes to 0 would be the same as sin(x)/x as x goes to infinity, not 0.
 
  • #3


sin(1/x)/(1/x) as x goes to 0 would be the same as sin(x)/x as x goes to infinity, not 0.

so it will be of the form sin(x)/x
applying L'hopitals rule :
cos(x)/1
= cos(x)
as you said x goes to infinity so what will be cos(∞)?
 
  • #4
D H
Staff Emeritus
Science Advisor
Insights Author
15,393
685


so it will be of the form sin(x)/x
applying L'hopitals rule :
cos(x)/1
= cos(x)
as you said x goes to infinity so what will be cos(∞)?
It's undefined, and all that that means is that you can't use l'Hôpital's rule here.

You can use the squeeze theorem on sin(x)/x as x→∞. sin(x) is bounded from above by +1, from below by -1. Given that, what does the squeeze theorem say about sin(x)/x as x→∞?
 
  • #5


It's undefined, and all that that means is that you can't use l'Hôpital's rule here.

You can use the squeeze theorem on sin(x)/x as x→∞. sin(x) is bounded from above by +1, from below by -1. Given that, what does the squeeze theorem say about sin(x)/x as x→∞?
ok ....got your point....its 0
thx a lot
 
  • #6
mathman
Science Advisor
7,901
460


so it will be of the form sin(x)/x
applying L'hopitals rule :
cos(x)/1
= cos(x)
as you said x goes to infinity so what will be cos(∞)?
L'Hopital's rule doesn't apply, since |sin(x)| is bounded while the denominator x becomes infinite.
 

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