Answer
7
Work Step by Step
$S_{n}=\dfrac {a_{1}+a_{n}}{2}\times n\Rightarrow a_{1}=\dfrac {2S_{n}}{n}-a_{n}\Rightarrow a_{1}=\dfrac {2\times S_{20}}{20}-a_{20}=\dfrac {2\times 1090}{20}-102=7$
Precalculus (6th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
013421742X
ISBN 13:
978-0-13421-742-0
Chapter 11 - Further Topics in Algebra - 11.2 Arithmetic Sequences and Series - 11.2 Exercises - Page 1025: 61
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