Answer
please see step-by-step
Work Step by Step
See p.711:
The sum of the first $n$ terms of a sequence is represented by the summation notation
$\displaystyle \sum_{i=1}^{n}a_{j}=a_{1}+a_{2}+a_{3}+a_{4}+\cdots+a_{n}$,
where $i$ is the index of summation,
$n$ is the upper limit of summation, and
1 is the lower limit of summation.
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For example, $\displaystyle \sum_{k=1}^{4}2k$
is the sum of the first 4 terms of the sequence whose general term is
$a_{n}=2n\quad \qquad (a_{k}=2k)$
This sequence is written as
2,4,6,8,10,12,....
The sum of the first four terms:
$\displaystyle \sum_{k=1}^{4}2k= 2+4+6+8=20$
