I'm not too sure how to prove that. I may have an answer but i'm not sure if it is correct. This is what i have:

(n squared - n) / 2 <= C * n squared 1/2 n squared - 1/2 n <= (a + 1) n squared

so if n >= 1.5 then 1/2n <=n squared, so..

1/2 n squared + n squared <= 2 1/2 n squared

thus proving there is a C that can be an upper bound for the T(n) equation and is = O(n squared)

yes/no?