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## 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.