just an intermediate arithmetic sum
 

Hi Mersenneforum,


The trick for adding consecutive numbers
in a series
can be to pair up the largest and the smallest
then
pair up the next largest and smallest
and then you either have one left in the middle
or you don't

So the sum of the counting numbers from 1 to r is

r times the quantity r+1 all over 2.

To compress

we have
r(r+1)/2
This expression is well known by those that well know it.
Regards,
Matt