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Calculus Level 1

lim x sin x x = ? \lim_{x\to\infty} \dfrac{\sin x}x = \, ?


The answer is 0.

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7 solutions

Damien LaRocque
Feb 21, 2016

We can use the sandwich theorem. We know that: 1 sin ( x ) 1 -1\le \sin { \left( x \right) } \le 1

Thus, lim x 1 x lim x sin ( x ) x lim x 1 x \lim _{ x\rightarrow \infty }{ \frac { -1 }{ x } } \le \lim _{ x\rightarrow \infty }{ \frac { \sin { \left( x \right) } }{ x } } \le \lim _{ x\rightarrow \infty }{ \frac { 1 }{ x } }

Hence, 0 lim x sin ( x ) x 0 0\le \lim _{ x\rightarrow \infty }{ \frac { \sin { \left( x \right) } }{ x } } \le 0

Rishabh Jain
Feb 5, 2016

The answer is 1..(obviously not since x \rightarrow \infty (not 0) ). Notice that as x \rightarrow \infty , 1/x tends to zero while sin x oscillates between 1 and -1. Hence, lim x 1 x × sin x \large \lim_{x\rightarrow \infty} \frac{1}{x} \times \sin x = 0 × (a finite number ) =0 \times \text{ (a finite number )} = 0 ~~~~~~~~~~~~~~~~~~\Large=\boxed 0

Limit does not exist. Use Taylor expansion.

Sharma Kunapalli - 5 years, 3 months ago

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Taylor expansion is an approximation about a point. So the expansion about X=0 gives no information about the function about infinity.

Wesley Chow - 5 years, 3 months ago

See oscillating limits on this page

Rishabh Jain - 5 years, 3 months ago

Limit exists, apply squeeze theorem or Damien LaRocque's solution is perfect and if you don't still beleieve it use Wolfram alpha.

Guillermo Templado - 5 years, 3 months ago
Xavier Mendoza
Feb 21, 2016

The limit that doesn't actually exist is sin (1/x). Sin x doesn't go further from -1,1 when x grows indefinitely, and the denominator grows indefinitely, which means that the number is reaching 0. Solution: 0.

Krishna McKay
Dec 8, 2017

As the numerator, sin(x), approaches infinity, it always stays within -1 and 1. But as the denominator, x, approaches infinity, it gets larger and larger. The numerator gets divided by larger and larger numbers, resulting in it constantly growing smaller (when the top is positive) or larger (when the topis negative) and consequently reaching zero

Cameron Elwardt
Jun 6, 2016

L'hopital's rule. The limit = 0.

How ??? Explain how l'opital's rule is applied here plz ...

Ibrahem Abd El Ghany - 5 years ago

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L'hopital's rule: When you have an undefined number, as in this case (sin(0)/0=infinity) you take the limit of the derivative of the top divided by the derivative of the bottom. In this case, let f(x)=the numerator and g(x)=the denominator. Then you take the limit as x approaches infinity of f'(x)/g'(x) which in this case would be cos(infinity)/1=0/1=0.

Sorry, I have no idea how to use LaTeX.

Cameron Elwardt - 5 years ago

You can't use L^hopital's rule here because the limit as approches infiny of cos(x) isn't 0, that limit is undefined.

Ofek Tevet - 1 year, 2 months ago
Joji Thomas
Feb 21, 2016

sinx is always b/w -1 and 1 so it is a finite number. also when we apply limits,x=(infinity)=1/0 which will give the solution a finite number*0=0 hence the answer

Rohan Gomes
Feb 21, 2016

Actually the limit doesn't exist

It does and it is zero

Mirco Soffritti - 5 years, 3 months ago

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