1. ## Cumulative Sum

What would be the formula to come up with a cumulative sum? For example, x=100 and # periods=3 so that x+2x+3x=600. But I want to be able to change the # periods and x to calculate the cumulative sum. Thank you!

2. ## Re: Cumulative Sum

$x + 2x + \dots + n x = \displaystyle \sum \limits_{k=1}^n ~k x = x \sum \limits_{k=1}^n ~k = x \dfrac{n(n+1)}{2}$

where $x$ is whatever you want and $n$ is what you're calling the number of periods.

3. ## Re: Cumulative Sum

Thank you so much!

4. ## Re: Cumulative Sum

That is, by the way, once you have factored out "x", an "arithmetic series": Arithmetic Series -- from Wolfram MathWorld.