sum(n)=\sum_{i=0}^n i =\frac{n^2 + n}{2} < \frac{n^2 + n^2}{2} = n^2 4*sum(n/2)= =\frac{n^2 + 2n}{2} = sum(n)+\frac{n}{2} \frac{\frac{n}{2}}{sum(n)} = \frac{n}{n^2+n} ->0