Answer
$\sum\limits_{i=1}^{n}{{{a}_{i}}}=\underline{{{a}_{1}}}+\underline{{{a}_{2}}}+\underline{{{a}_{3}}}+.....+\underline{{{a}_{n}}}$. In this summation notation, $i$ is called the index of summation, $n$ is the upper limit of summation, and $1$ is the lower limit of summation.
Work Step by Step
We know that the sum of the first n terms of a sequence is represented by the summation notation$\sum\limits_{i=1}^{n}{{{a}_{i}}}={{a}_{1}}+{{a}_{2}}+{{a}_{3}}+.....+{{a}_{n}}$. Here, $i$ stands for index of summation, $n$ is the upper limit of summation, and $1$ is the lower limit of summation. Any letter can be used to represent the index of summation and the lower limit of summation can be an integer, but not $1$.
