Posted by Tiffany on Saturday, June 7, 2014 at 8:04pm.
1+2+3+...+n=(1+n)+(2+(n1))+(3+(n2))+...
=(1+n)+(1+n)+(1+n)+...
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 greatestinteger function.