Answer
D
Work Step by Step
In the sequence, we add n each time.
$a_{1}=1$
$a_{2}=1+2$
$a_{3}=1+2+3$
The formula of adding successive positive integers is $n*(n+1)/2$
Algebra 2 (1st Edition)
by Larson, Ron; Boswell, Laurie; Kanold, Timothy D.; Stiff, Lee
Published by
McDougal Littell
ISBN 10:
0618595414
ISBN 13:
978-0-61859-541-9
Chapter 12 Sequences and Series - 12.1 Define and Use Sequences and Series - 12.1 Exercises - Skill Practice - Page 798: 27
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