Answer
arithmetic,
$d= -8$,
$S_n= -n(4n-2)$.
Work Step by Step
1. Based on the given sequence, we can determine that it is arithmetic,
2. We can find the common difference as $d=a_2-a_1=-8$,
3. We can find the sum of the first n terms as $S_n=\frac{n(2a_1+(n-1)d)}{2}=\frac{n(2(-2)-8(n-1))}{2}=-n(4n-2)$.
