Answer
a) The two sums are the same in what they represent but only differ in indices.
b) The two sums are different.
Work Step by Step
a) The two sums are the same in what they represent but only differ in indices. They both represent a summation of $n$ terms of the sequence $a_{i}$ or $a_{j}$, which looks like the following:
$a_{1}$+$a_{2}$+$a_{3}$ +...+$a_{n}$ = $\Sigma a_{x}$, here $x$ is either $i$ or $j$ .
b) The first notation means the same as part a, being the following:
$a_{1}$+$a_{2}$+$a_{3}$ +...+$a_{n}$ = $\Sigma a_{i}$ , that is, the sum of all terms of $a_{i}$ which is influenced by the index i progressing from $1$ to $n$. On the other hand, the notation where the term is $a_{j}$ and the index of summation symbol is $i$ starting from $1$ going to $n$ is equal to $n*a_{j}$ .
