Answer
$S_{n}$ = $\frac{n}{2}$($a_{1}$ + $a_{n}$)
$a_{1}$ is the first term,
$a_{n}$ is the last term
$a_{n}$ = $a_{1}$ + (n - 1) d
Work Step by Step
The sum $S_{n}$ = $\frac{n}{2}$($a_{1}$ + $a_{n}$)
Where
$a_{1}$ is the first term, and $a_{n}$ is the last term
$a_{n}$ = $a_{1}$ + (n - 1) d
