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Your expression is
$$ E = \frac{cn^2}{\log \frac{n(n+1)}{2}}$$
where $c$ is some constant. The simple upper bound for $E$ is
$$ E\le c n^2$$
which implies that $\mathcal{O}(n^2)$. For a better bound
$$E = \frac{cn^2}{\log \frac{n(n+1)}{2}} = \frac{cn^2}{ 2 \log n + \log n - \log 2 } $$
Now it is an easy verification that $E$ is $\mathcal{O}(\frac{n^...
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