Is it possible to solve this finding the result of the summatory and then, the result of the product?
$\prod_{i=1}^{2}\sum_{j=1}^{n} ij$
I already know that $\sum_{j=1}^{n} ij = i\frac{n(n+1)}{2}$, so I would have to simply find $\prod_{i=1}^{2} i\frac{n(n+1)}{2} $?
Yes, that is correct. Adding parentheses for clarity, we have $$ \prod_{i=1}^2 \sum_{j=1}^n ij = \prod_{i=1}^2 \left(\sum_{j=1}^n ij\right). $$