# Thread: Finding the sum of an infinite sequence

1. ## Finding the sum of an infinite sequence

Hey all, hope you're doing well. Here's the bonus question that's been bugging me for a while now:

For anyone who can point me on the right track, your help is much appreciated.

- Dave

(NOTE: I know the answer is ln(2) from Maple; I just don't know how to get a ln from that sum)

2. Originally Posted by urnidiot
Hey all, hope you're doing well. Here's the bonus question's that's been bugging me for a while now:

For anyone who can point me on the right track, your help is much appreciated.

- Dave

(NOTE: I know the answer is ln(2) from Maple; I just don't know how to get a ln from that sum)
You should know the definition of a Riemann sum for this:
$\displaystyle \lim_{n \to \infty} \sum_{k=1}^{k=n} \frac1{n+k} = \lim_{n \to \infty}\frac1{n} \sum_{k=1}^{k=n} \dfrac1{1+\dfrac{k}{n}} = \int_{0}^{1} \frac1{1+x} \, dx$

3. Thanks for the reply! However, I'm still not sure how to get from the second equation to the third - if you could elaborate that a little, I'd be very grateful.

-Dave

4. Originally Posted by urnidiot
Thanks for the reply! However, I'm still not sure how to get from the second equation to the third - if you could elaborate that a little, I'd be very grateful.

-Dave
Well, $\displaystyle \int_0^1 \frac{dx}{x+1} =_{t=x+1} \int_1^2 \frac{dt}{t} = \ln 2$.

5. Thanks man, but that's not the part I'm having trouble understanding; it's the step before that to get to the integral.

-Dave

6. Originally Posted by urnidiot
Thanks man, but that's not the part I'm having trouble understanding; it's the step before that to get to the integral.

-Dave
As I said, you should know the definition of a Riemann sum to understand that. Do you know what a Riemann sum is?

I have used the following result:
$\displaystyle \lim_{n \to \infty} \frac1{n} \sum_{k=0}^{k=n} f\left(a + \frac{k(b-a)}{n}\right) = \int_a ^b f(x) \, dx$