College Algebra (10th Edition)

Published by Pearson
ISBN 10: 0321979478
ISBN 13: 978-0-32197-947-6

Chapter 9 - Section 9.1 - Sequences - 9.1 Assess Your Understanding - Page 647: 59


$1+2+3+\displaystyle \cdots+20=\sum_{k=1}^{20}k$

Work Step by Step

$1+2+3+\cdots+20=$ There are 20 terms, count them with index k: $\displaystyle \sum_{k=1}^{20}$(...) The first term (when k=1) is 1, The second term (when k=2) is 2, The kth is k so $1+2+3+\displaystyle \cdots+20=\sum_{k=1}^{20}k$
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