Question: what is the formula n(n+1)/2?
Answer: As @betlista has said, n(n-1)/2 is the sum of the first (n-1) numbers, that is 1 + 2 + 3 + 4 + .......... + (n-1) Now one might think that there is not much use for this formula, but when you do some research, you can find interesting uses for it. @ betlista has explained a few uses.