Answer
$\displaystyle \sum_{i=1}^{8}(i+4)$
Work Step by Step
From the given terms, we observe the pattern
$5=4+1$
$6=4+2$
$7=4+3$
$...$
$12=4+8$
So, there are 8 terms in all, counting each as $a_{i}, i=1,2,...8,$
(lower limit is 1, upper is 8)
Sum = $\displaystyle \sum_{i=1}^{8}(i+4)$
