Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 10: Infinite Sequences and Series - Section 10.1 - Sequences - Exercises 10.1 - Page 571: 116

Answer

$\{a_{n}\}$ diverges to infinity.

Work Step by Step

For any given (large) positive number M, we can find an N such that $n\gt N\Rightarrow a_{n}\gt M:$ \begin{align*} N-\displaystyle \frac{1}{N}&\gt M \\ \displaystyle \frac{N^{2}-1}{N}&\gt M \\ N^{2}-1&\gt MN\\ N^{2}-MN-1&\gt 0 \end{align*} The function $f(x)=x^{2}-Mx-1$ has two distinct zeros as the discriminant $b^{2}-4ac=M^{2}+4$ is positive. Let $x_{2}$ be the greater of the two zeros. Its graph is a parabola that opens up, so, for $x\gt x_{2},\ f(x)$ is positive. We take N to be the first positive integer greater than $x_{2}$ and we are guaranteed that $ n\gt N\Rightarrow a_{n}\gt M$, because $n^{2}-Mn-1\gt 0$ Thus, by definition, $\{a_{n}\}$ diverges to infinity.
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