n + (n-1) + (n-2) + (n-3) + ... + (1) = n(n+1)/2

as n becomes large n becomes negligable compared to n^2 so:

n + (n-1) + (n-2) + (n-3) + ... + (1) = n(n+1)/2 = (n^2)/2 +n/2 ~ (n^2)/2,

or in big-O notations:

..................................................= O(n^2).

RonL