# Trying to find the infinite sum of e^-x using integration

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1. Oct 30, 2014

Hello,

I am well aware of the ratio method, and the sum = 1/(1-r) but I want to try this method.

I am trying to understand this:

$\displaystyle \sum_{n=1}^{\infty} e^{-n}$ using integrals, what I have though:

$= \displaystyle \lim_{m\to\infty} \sum_{n=1}^{m} e^{-n} = \displaystyle \lim_{m\to\infty} \frac{1}{m}\sum_{n=1}^{m} me^{-n}$

So, suppose this is an right-hand Riemann sum, with $m$ *Equal* subintervals.

$f(x_i) = me^{-n}$ represents the *height* of the function, we will have the integral for.

$\Delta(x) = \frac{1}{m}$

But, How can this be represented as an integral?

Thanks!

Last edited by a moderator: Oct 30, 2014
2. Oct 30, 2014

### quantumtimeleap

Hi! I'm new here at PF ^^

I believe you're not going to be able to use the Riemann integral definition with a summation of that form. You are trying to use 1/m as the $\Delta x$ in the definition of the Riemann sum:

$\sum_{n=0}^\infty f(x) \Delta x$

However 1/m cannot be a difference of two coordinates $x_{n} - x_{n-1}$