Answer
$20$
Work Step by Step
Recall the explicit formula to calculate the sum of a finite arithmetic series.
$S_{n}=\dfrac{n}{2} (a_1+a_n)$
where; $a_n = n^{\text{th}} term; \\ a_1 = \ first \ term;
\\ n =\ Number \ of \ Terms ; \ \\ r =\ Common \ Ratio $
From the given quadratic series, we have:
$n=4, a_{1}=2; a_{4}=8$
Thus, substituting these values into the formula above gives:
$S_{4}=\dfrac{4}{2} (2+8) \\=(2)(10)$
Therefore, $S_4=20$
