Finding a formula for a difficult sequence

[torquerotates]

Homework Statement

I'm trying to find a formula for 1, 3, 6, 10 ... for n=2,3,4,...

The Attempt at a Solution

Well I know that the difference between each number is increasing by 1. For example, the difference between 1 and 3 is 2, between 3 and 6 is 3, between 6 and 10 is 4 and so on. But, I just don't see a formula.

 

[rochfor1]

Hi! $$a_n = \sum_{ k = 1 }^n k = \frac{ n ( n + 1 ) }{ 2 }$$

 

[torquerotates]

For some reason, I can't read that.