I learned this formula before high school. It was considered trivial. The demonstration was to write the sums 1+2+...N and N+(n-1)+...+1 and add them up positi

I learned this formula before high school. It was considered trivial. The demonstration was to write the sums 1+2+...N and N+(n-1)+...+1 and add them up position wise. The sum comes n*(n+1)/2 (divide by 2 for the two sums).