How many copies of n+1 are in there in the second line?
with n=6 and n=7
Number Theory - Steve, Saturday, June 7, 2014 at 10:47pm
Since the n numbers are divided up in pairs, I'd say n/2 copies.
Hence the formula for the sum: n(n+1)/2
Technically, that's [n+1]/2 where  is the greatest-integer function.