College Algebra (10th Edition)

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

Chapter 9 - Section 9.2 - Arithmetic Sequences - 9.2 Assess Your Understanding - Page 654: 28


$a_{80} =-390$

Work Step by Step

RECALL: (1) The $n^{th}$ term of an arithmetic sequence can be found using the formula: $a_n = a_1 + (n-1)d$ where $a_1$ = first term and $d$ = common difference (2) The common difference $d$ can be found by subtracting any term to the next term of the sequence: $d=a_n - a_{n-1}$ The given sequence has: $a_1=5$; $d=0-5=-5$ Substitute these values into the formula for the $n^{th}$ term to obtain: $a_n=a_1 + (n-1)d \\a_n=5+(n-1)(-5) \\a_n=5+(-5)(n-1) \\a_n=5-5(n-1)$ To find the 80th term, substitute $80$ for $n$ to obtain: $a_{80} = 5-5(80-1) \\a_{80}=5-5(79) \\a_{80}=5-395 \\a_{80} =-390$
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