Answer
$S_4=-40$
Work Step by Step
Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, the sum of the first $
4
$ terms of the sequence $
-4,-8,-12,...
$ is
\begin{array}{l}
S_4=\dfrac{4}{2}(2\cdot(-4)+(4-1)(-4))\\\\
S_4=2(-8-12)\\\\
S_4=-40
.\end{array}
Intermediate Algebra (6th Edition)
by Martin-Gay, Elayn
Published by
Pearson
ISBN 10:
0321785045
ISBN 13:
978-0-32178-504-6
Chapter 11 - Section 11.4 - Partial Sums of Arithmetic and Geometric Sequences - Exercise Set - Page 658: 6
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