Limitless

Calculus Level 3

lim x 1 ( 1 x ) ( 1 x 3 ) ( 1 x n ) ( 1 x ) n 1 = ? \large \displaystyle\lim_{x\rightarrow 1} \frac{(1-\sqrt{x})\cdot (1-\sqrt[3]{x})\cdots (1-\sqrt[n]{x})}{(1-x)^{n-1}} =\, ?


The answer is 0.

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1 solution

Adrian Castro
Jan 8, 2016

The limit above simplifies to lim x 1 1 x = 0 \lim_{x\rightarrow{1}}\;1-\sqrt{x}=\boxed{0} because 1 x n = 1 x 1-\sqrt[n]{x}=1-x for all n when substituted for x = 1 x=1

Nice observation! There are other ways to get the solution without substituting. Try to get this: lim n 1 n ! \displaystyle\lim_{n\rightarrow \infty} \frac{1}{n!} . You could also use "The Squeeze Theorem".

Aleksa Radovanović - 5 years, 5 months ago

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Thanks man

Adrian Castro - 5 years, 5 months ago

I substituted n=1.

A Former Brilliant Member - 5 years, 5 months ago

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You can't solve the problem only with substituting n=1, but you could proove by induction that it works for all n.

Aleksa Radovanović - 5 years, 5 months ago

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Here is my proof by induction: Let P n P_{n} be the proposition that lim x 1 ( 1 x ) ( 1 x 3 ) ( 1 x n ) ( 1 x ) n 1 = 0 \large \displaystyle\lim_{x\rightarrow 1} \frac{(1-\sqrt{x})\cdot (1-\sqrt[3]{x})\cdots (1-\sqrt[n]{x})}{(1-x)^{n-1}} =\,0 for positive integer n.

For base case n=1:

LHS= lim x 1 ( 1 x ) ( 1 x ) 0 = 0 \lim_{x \rightarrow 1}\frac{(1-\sqrt{x})}{(1-x)^{0}}=0

Assume P k P_{k} is true for some positive integer k.

Then, when n = k + 1 n=k+1 :

LHS= lim x 1 ( 1 x ) ( 1 x 3 ) ( 1 x k ) ( 1 x k + 1 ) ( 1 x ) k = lim x 1 ( 1 x ) ( 1 x 3 ) ( 1 x k ) ( 1 x ) k 1 1 x k + 1 x k \large \displaystyle \lim_{x\rightarrow 1} \frac{(1-\sqrt{x})\cdot (1-\sqrt[3]{x})\cdots (1-\sqrt[k]{x})(1-\sqrt[k+1]{x})}{(1-x)^{k}} = \large \displaystyle\lim_{x\rightarrow 1} \frac{(1-\sqrt{x})\cdot (1-\sqrt[3]{x})\cdots (1-\sqrt[k]{x})}{(1-x)^{k-1}}\cdot \frac{1-\sqrt[k+1]{x}}{x^{k}}

= lim x 1 ( 1 x ) ( 1 x 3 ) ( 1 x k ) ( 1 x ) k 1 lim x 1 1 x k + 1 x k \large \displaystyle = \lim_{x\rightarrow 1} \frac{(1-\sqrt{x})\cdot (1-\sqrt[3]{x})\cdots (1-\sqrt[k]{x})}{(1-x)^{k-1}}\cdot \lim_{x \rightarrow 1}\frac{1-\sqrt[k+1]{x}}{x^{k}}

= 0 \large \displaystyle =0

Hence, by Mathematical Induction, since P 1 P_{1} , P k P_{k} , P k + 1 P_{k+1} are true, the proof is complete.

A Former Brilliant Member - 5 years, 5 months ago

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@A Former Brilliant Member That's it :)

Aleksa Radovanović - 5 years, 5 months ago

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@Aleksa Radovanović I'm not sure that induction can be applied here. Since the numerator starts with ( 1 x ) (1 - \sqrt{x}) the case n = 1 n = 1 doesn't really make sense. For n = 2 n = 2 the limit is

lim x 1 1 x 1 x = lim x 1 1 x ( 1 x ) ( 1 + x ) = 1 2 . \lim_{x \rightarrow 1} \dfrac{1 - \sqrt{x}}{1 - x} = \lim_{x \rightarrow 1} \dfrac{1 - \sqrt{x}}{(1 - \sqrt{x})(1 + \sqrt{x})} = \dfrac{1}{2}.

In general, lim x 1 1 x 1 n 1 x = 1 n \lim_{x \rightarrow 1} \dfrac{1 - x^{\frac{1}{n}}}{1 - x} = \dfrac{1}{n} for any non-zero integer.

So the given limit can be written as k = 2 n 1 k = 1 n ! \displaystyle\prod_{k=2}^{n} \dfrac{1}{k} = \dfrac{1}{n!} , which would only go to 0 0 as n . n \rightarrow \infty.

Brian Charlesworth - 5 years, 5 months ago

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@Brian Charlesworth Yes. I agree with your logic.

A Former Brilliant Member - 5 years, 5 months ago

@Brian Charlesworth Yes, did same. Also, putting n=1 does not make sense here.

Pulkit Gupta - 5 years, 5 months ago

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