Infinite Limit?

Calculus Level 3

lim n ( 1 e 1 2 + 1 e 2 3 + 1 e 3 4 + + 1 e n 1 n ) = ? \large \lim_{n\to\infty} \left( \frac1{e^{\frac12}} + \frac1{e^{\frac23}} + \frac1{e^{\frac34}} + \ldots + \frac1{e^{\frac{n-1}n}} \right ) = \ ?

e e \infty e 2 e^{2} 1 e \frac{1}{e}

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3 solutions

Satyajit Mohanty
Jul 7, 2015

Moderator note:

There's a much simpler approach.

Hint : Consider a n = e n 1 n a_n = e^{-\frac{n-1}n} . What happens when we consider the limit lim n a n \displaystyle \lim_{n\to\infty} a_n ?

@Challenge Master , the value of that would be 1 e \frac{1}{e} :)

Satyajit Mohanty - 5 years, 11 months ago

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Hey bro which is greater half or one third.... .

Suyash Harsh - 5 years, 11 months ago

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Wrong solution,kindly review it

Suyash Harsh - 5 years, 11 months ago

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@Suyash Harsh Hi @Suyash Harsh , Thank you for pointing out my silly mistake. The solution is definitely correct, I only had some typo-error in the second line. Actually, it'd be 1 n < 1 n 1 \frac{1}{n} < \frac{1}{n-1} in the second line. Everything is correct from the third line onwards. :) I've edited the second line of the solution. :)

Satyajit Mohanty - 5 years, 11 months ago

L = 1 e 1 / 2 + 1 e 2 / 3 + 1 e 3 / 4 + . . . L > 1 e + 1 e + 1 e + . . . \begin{aligned} L & = \frac 1{e^{1/2}}+ \frac 1{e^{2/3}}+ \frac 1{e^{3/4}}+...\\ L & > \frac 1e + \frac 1e+ \frac 1e+...\end{aligned}

Since the RHS diverges and L > L> RHS, L L diverges, L \implies L \to \boxed{\infty}

Tanishq Varshney
Jul 10, 2015

lim n ( 1 e 1 1 / 2 + 1 e 1 1 / 3 + . . . + 1 e 1 1 / n ) \large{\displaystyle \lim_{n\to \infty} (\frac{1}{e^{1-1/2}}+\frac{1}{e^{1-1/3}}+...+\frac{1}{e^{1-1/n}})}

1 e lim n r = 2 n e 1 r \large{\frac{1}{e} \displaystyle \lim_{n\to \infty} \sum_{r=2}^{n} e^{\frac{1}{r}}}

Now

lim n e 1 n = 1 \large{\displaystyle \lim_{n\to \infty} e^{\frac{1}{n}}=1}

and e 1 / 2 = 1 p o i n t s o m e t h i n g e 1 / 3 = 1 p o i n t s o m e t h i n g e^{1/2}=1~point ~something\quad e^{1/3}=1~ point~something and so on

1 e ( 1. a b c + 1. p q r + 1. x y z + . . . . . i n f i n i t e t e r m s ) \frac{1}{e}(1.abc+1.pqr+1.xyz+.....infinite~terms)

Answer is \infty

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