Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 11 - Infinite Series - 11.2 Summing and Infinite Series - Preliminary Questions - Page 546: 3


The formula is not valid in this case.

Work Step by Step

This is a geometric series in which $c=1$ and $r=3$ with the sum $$\frac{1}{1-3}=-\frac{1}{2}.$$ It is impossible for the sum to be negative, since all the terms are positive. The formula is not valid because we have $r=3$ and we must have $|r|\lt 1$ to use the formula.
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