Answer
$550$
Work Step by Step
Here, $a_1=-1, a_{20}=56$
Now, we have
$S_n=\dfrac{n}{2}[a_1+a_n]$
Now,
$S_{20}=\dfrac{20}{2}[-1+56]$
or, $=550$
Intermediate Algebra for College Students (7th Edition)
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0-13417-894-7
ISBN 13:
978-0-13417-894-3
Chapter 11 - Test - Page 871: 7
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