# Limit of a sum

## Homework Statement

lim n->infinity of a[n] = ?

|x| > 1

## The Attempt at a Solution

Well, actually i figured out that the sequence converges, and i've tried to solve it using riemannian sums and integrals but no success.

I've also tried to solve the sum and then calculate the limit but i had no idea where to start from.

May anyone suggest me a starting point?

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I think we can find the sum of a series only if it is a geometric series or a telescoping sum, and from a quick glance this doesn't look like either one.
Otherwise we would need some math software to approximate the sum.

Dick
Homework Helper

## Homework Statement

lim n->infinity of a[n] = ?

|x| > 1

## The Attempt at a Solution

Well, actually i figured out that the sequence converges, and i've tried to solve it using riemannian sums and integrals but no success.

I've also tried to solve the sum and then calculate the limit but i had no idea where to start from.

May anyone suggest me a starting point?

Hint: think about what happens when you take derivatives of a geometric series.

Last edited by a moderator:

## Homework Statement

lim n->infinity of a[n] = ?

|x| > 1

## The Attempt at a Solution

Well, actually i figured out that the sequence converges, and i've tried to solve it using riemannian sums and integrals but no success.

I've also tried to solve the sum and then calculate the limit but i had no idea where to start from.

May anyone suggest me a starting point?

use the ratio test. ignore the summation symbol for now.

After using ratio test. see if you can seperate the functions so you have "(function 1) + or - (function 2)"

if you can do that, then you could probably telescope to solve the sum. If not, then it could be a geometric series, which should be easier to test (if it is or not) after u get it to a simplified form.

If that doesnt work you should retry it but changing the index. i.e do an index shift, sub k=K+1 (them summation will start at 0), index will be K, not k-1. ratio test should be easier

(also, isnt this question supposed to be something like, for what values of x or k, is this covnergant?)

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Mark44
Mentor
use the ratio test. ignore the summation symbol for now.

After using ratio test. see if you can seperate the functions so you have "(function 1) + or - (function 2)"

if you can do that, then you could probably telescope to solve the sum. If not, then it could be a geometric series, which should be easier to test (if it is or not) after u get it to a simplified form.

If that doesnt work you should retry it but changing the index. i.e do an index shift, sub k=K+1 (them summation will start at 0), index will be K, not k-1. ratio test should be easier

(also, isnt this question supposed to be something like, for what values of x or k, is this covnergant?)
The OP has already determined that the series converges. He/she is now trying to figure out what the series converges to.

Dick
Homework Helper
My hint still stands. Figure out the sum of r^(k+1). Differentiate twice and put r=1/x.

I figured out how to do it differntiating a geometric series after Dick gave me that hint but i hadn't time to do it yesterday, i will come back later with the solution.

HallsofIvy