Algebra and Trigonometry 10th Edition

Published by Cengage Learning
ISBN 10: 9781337271172
ISBN 13: 978-1-33727-117-2

Chapter 11 - 11.2 - Arithmetic Sequences and Partial Sums - 11.2 Exercises - Page 786: 54



Work Step by Step

$d=70-75=65-70=-5$ $a_n=a_1+(n-1)d$ $a_{25}=75+(25-1)(-5)$ $a_{25}=75+24(-5)=75-120$ $a_{25}=-45$ $S_n=\frac{n}{2}(a_1+a_n)$ $S_{25}=\frac{25}{2}(75-45)=\frac{25}{2}(30)=375$
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