Answer
See below.
Work Step by Step
The sum of the first $n$ terms is $n$ times the sum of the first and the nth term divided by $2$. So it is $S_n=\frac{n(a_1+a_n)}{2}$
College Algebra (6th Edition)
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0-32178-228-3
ISBN 13:
978-0-32178-228-1
Chapter 8 - Sequences, Induction, and Probability - Exercise Set 8.2 - Page 726: 75
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