umm.. im not really good at english but i hope i can help you.
this is a sequence of triangular numbers.
I'll give you a general term :
n(n+1)
2
n is the number of numbers you have in your sequence, like:
1+2+3+4+5+6+7
then you'll put it like this:
7(7+1)
2
hope you'll understand xdddddddd
