5 + 8 = 17 $$
So, for n=4, the sum is 17.
Now, let's write out the general formula for the sum:
$$ \sum_{k=1}^n \frac{k^2}{2} $$
This sum represents the sum of the squares of the first n natural numbers, each divided by 2.
So, the sum can be written as:
$$
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