## College Algebra (10th Edition)

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