College Algebra 7th Edition

Published by Brooks Cole
ISBN 10: 1305115546
ISBN 13: 978-1-30511-554-5

Chapter 8, Sequences and Series - Section 8.2 - Arithmetic Sequences - 8.2 Exercises: 51

Answer

$S_{20}=1010$

Work Step by Step

RECALL: The sum of the first $n$ terms ($S_n$) of an arithmetic sequence is given by the formula: $S_n=\dfrac{n}{2}\left[2a+(n-1)d\right]$ where $a$ = first term $d$ = common difference The given arithmetic sequence has: $a=3 \\d=5$ Thus, to find the sum of the first 20 terms, substitute the given values to the formula above to obtain: $S_n=\dfrac{n}{2}[2a+(n-1)d] \\S_{20}=\dfrac{20}{2}[2(3) + (20-1)5] \\S_{20}=10[6+19(5)] \\S_{20}=10(6+95) \\S_{20}=10(101) \\S_{20}=1010$
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