Answer
$-269$
Work Step by Step
We know that $a_n=a_1+(n-1)d.$
where $d$=common difference and $a_1$= the first term
Here $a_1=1$ and $d=a_2-a_1=-2-1=-3$
Hence, here we have
$a_n=1+(n-1)\cdot2.$
Therefore $a_{90}=1+(90-1)\cdot(-3)\\=-269.$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.2 Arithmetic Sequences - 12.2 Assess Your Understanding - Page 814: 27
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