An Intriguing 'Limits' Problem #2

Calculus Level 5

lim n ( i = 0 2 n ( 2 n i ) ) 2 n 1 4 n 3 n = ? \large{\lim_{n \to \infty} \left( \sqrt{\prod_{i=0}^{2n} {2n \choose i}} \ \right)^{\frac{2n-1}{4n^3 - n}} = \ ?}


The answer is 2.

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2 solutions

Satyajit Mohanty
Jul 23, 2015

We know 2 2 n = i = 0 2 n ( 2 n i ) 2^{2n} = \displaystyle \sum_{i=0}^{2n} {2n \choose i}

We have by A.M - G.M Inequality:

i = 0 2 n ( 2 n i ) 2 n + 1 ( i = 0 2 n ( 2 n i ) ) 1 2 n + 1 \dfrac{\displaystyle \sum_{i=0}^{2n} {2n \choose i}}{2n+1} \geq \left(\prod_{i=0}^{2n} {2n \choose i} \right)^{\frac{1}{2n+1}}

2 2 n 2 n + 1 ( i = 0 2 n ( 2 n i ) ) 1 2 n + 1 \Rightarrow \dfrac{2^{2n}}{2n+1} \geq \left( \prod_{i=0}^{2n} {2n \choose i} \right)^{\frac{1}{2n+1}}

( 2 2 n 2 n + 1 ) 1 2 n ( i = 0 2 n ( 2 n i ) ) 1 2 n ( 2 n + 1 ) \Rightarrow \left( \dfrac{2^{2n}}{2n+1} \right)^{\frac{1}{2n}} \geq \left(\prod_{i=0}^{2n} {2n \choose i} \right)^{\frac{1}{2n(2n+1)}}

2 ( 2 n + 1 ) 1 2 n ( i = 0 2 n ( 2 n i ) ) 1 2 n ( 2 n + 1 ) \Rightarrow 2 \geq (2n+1)^{\frac{1}{2n}} \left(\prod_{i=0}^{2n} {2n \choose i} \right)^{\frac{1}{2n(2n+1)}}

Therefore, when n n tends to \infty ,

lim n ( 2 n + 1 ) 1 2 n ( i = 0 2 n ( 2 n i ) ) 1 2 n ( 2 n + 1 ) = lim n ( i = 0 2 n ( 2 n i ) ) 2 n 1 4 n 3 n = 2 \lim_{n \to \infty} (2n+1)^{\frac{1}{2n}} \left(\prod_{i=0}^{2n} {2n \choose i} \right)^{\frac{1}{2n(2n+1)}}= \lim_{n \to \infty} \left( \sqrt{\prod_{i=0}^{2n} {2n \choose i}} \ \right)^{\frac{2n-1}{4n^3 - n}} = \boxed{2}

This only gives 2 2 as an upper bound to the limit (assuming it to exist). You also need to show a lower bound.

Abhishek Sinha - 2 years, 5 months ago
Ramiel To-ong
Aug 14, 2015

nice solution, but i used table on that problem

This isn't a solution. If you want to say something, please add this as a comment to my solution!

Satyajit Mohanty - 5 years, 9 months ago

there is this same question but with my method it is coming (e)^(1/2) Plz help.

Dhairy Agrawal - 3 years, 4 months ago

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