If I were to simplify a summation that contains a formula that is not a constant and does not contain the index, what would it simplify down to?
For example, if were to simplify the following summation:
$$ \sum_{i=1}^{100} j $$
Would the result be equal to 100j or 0?
We have
$$\sum_{i=1}^{100}a_i=a_1+a_2+\cdots+a_{100}$$
In your example, $a_i=j$ for every $i$, and so
$$\sum_{i=1}^{100}a_i=\underbrace{j+j+\cdots+j}_{\text{100 times}}=100j$$