n(n-1)/2 expands to (n^2 -n) / 2, that is (n^2/2) - (n/2)
(n^2/2) and (n/2) are the two functions components, of which n^2/2 dominates.
Therefore, we can ignore the - (n/2) part.
From n^2/2 you can safely remove the /2 part in asymptotic notation analysis.
This simplifies to
n^2
Therefore yes, it is in O(n^2)
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