The Limits of Sine

Calculus Level 3

Does the following limit converge?

lim x 0 x 2 sin 1 x sin x \large \lim_{x\to 0 } \dfrac{ x^2 \sin \frac{1}{x} } { \sin x }

Does not converge Converges to a positive value Converges to a negative value Converges to 0

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2 solutions

Chew-Seong Cheong
Nov 15, 2016

lim x 0 x 2 sin 1 x sin x = lim x 0 sin 1 x x 2 sin x \begin{aligned} \lim_{x \to 0} \frac {x^2 \sin \frac 1x}{\sin x} & = \lim_{x \to 0} \sin \frac 1x \cdot \frac {x^2}{\sin x} \end{aligned}

We note that 1 sin 1 x 1 -1 \le \sin \frac 1x \le 1 is bounded and lim x 0 x 2 sin x = 0 \displaystyle \lim_{x \to 0} \dfrac {x^2}{\sin x} = 0 by L’Hôpital's rule , therefore, lim x 0 x 2 sin 1 x sin x = 0 \displaystyle \lim_{x \to 0} \frac {x^2 \sin \frac 1x}{\sin x} = \boxed{0}

Kushal Bose
Nov 15, 2016

lim x 0 x 2 s i n ( 1 / x ) s i n x = lim x 0 x s i n x × lim x 0 x s i n ( 1 / x ) = 1 × lim x 0 x s i n ( 1 / x ) \displaystyle \lim_{x \to 0} \frac{x^2 sin(1/x)}{sinx} \\ =\displaystyle \lim_{x \to 0} \frac{x}{sinx} \times \displaystyle \lim_{x \to 0} x sin(1/x) \\ =1 \times \displaystyle \lim_{x \to 0} x sin(1/x)

As x 0 x \rightarrow 0 then s i n ( 1 / x ) s i n ( ) sin(1/x) \to sin(\infty) which will oscillates between [ 1 , 1 ] [-1,1] .So this will be finite.Finite value multiplied with zero equals to zero.

So, the limit will be zero.

The last line could be clarified. Saying that "this will be finite" is meaningless because the limit of sin ( 1 / x ) \sin ( 1/x) does not exist, so it's unclear what you mean by sin \sin \infty .

You are actually using the squeeze theorem , that x sin x x |x \sin x | \leq |x| and hence the limit is 0.

Calvin Lin Staff - 4 years, 7 months ago

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But limit x s i n ( 1 / x ) xsin(1/x) exists

Kushal Bose - 4 years, 7 months ago

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Ah. Let me edit my comment to clarify.

Note: If you use \sin for the trigonometric function, that would make it easier to read.

Calvin Lin Staff - 4 years, 7 months ago

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@Calvin Lin So, I can edit my solution by writing squeeze theorem

Kushal Bose - 4 years, 7 months ago

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@Kushal Bose Yes, that is the rigorous step that you're missing.

Calvin Lin Staff - 4 years, 7 months ago

sin(1/x)/(1/x)=xsin(1/x)=??!

Patience Patience - 4 years, 7 months ago

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That is correct (with slight assumptions about the brackets).

Note that 1 1 x = x \frac{1}{ \frac{1}{x} } = x .

Calvin Lin Staff - 4 years, 7 months ago

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