1. The problem statement, all variables and given/known data How should I find the nth sum, of the series 2 + 5 + 9 + 14 + ... + n ? (the difference between terms increases by 1 each time) 2. Relevant equations 3. The attempt at a solution I only know how to sum geometric and arithmetic series and this is neither. Perhaps somebody could at least give me the name of this series so I can look it up on the net as a start? Thanks.