Answer
arithmetic,
$d=1$,
$S_n=\frac{n(n+11)}{2}$.
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 $d=a_2-a_1=7-6=1$,
3. We can find the sum of the first n terms as $S_n=\frac{n(a_1+a_n)}{2}=\frac{n(6+n+5)}{2}=\frac{n(n+11)}{2}$.
