University Calculus: Early Transcendentals (3rd Edition)

Published by Pearson
ISBN 10: 0321999584
ISBN 13: 978-0-32199-958-0

Chapter 9 - Section 9.1 - Sequences - Exercises - Page 489: 124

Answer

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

Work Step by Step

We interpret the sequence $1, -1,-5,-13,-29,$... as $(3-2), (3-4), (3-8), (3-16),...,(3-2^{n}),....$ $a_{1}=1\qquad (=-2^{n}+3$ when n=$1)$ $a_{2}=2(1)-3=-1\qquad (=-2^{n}+3$ when n=$2)$ $a_{3}=2(-1)-3=-5\qquad (=-2^{n}+3$ when n=$3)$ $a_{4}=2(-5)-3=-13\qquad (=-2^{n}+3$ when n=$4)$ $a_{5}=2(-13)-3=-29\qquad (=-2^{n}+3$ when n=$5)$ $...$ $a_{n}=-2^{n}+3$ We see that: $\qquad\{a_{n}\}$ is unbounded from below because $2^{n}$ grows without bound, $\qquad\{a_{n}\}$ is monotonic, nonincreasing This sequence is divergent. To check, use the definition for "diverges to negative infinity". Let m be a negative number. We want to find N such that $n\gt N\Rightarrow a_{n}\lt m$ $-2^{N}+3=m$ $3-m=2^{N} \quad $3-m is positive; we can take the logarithm $\log_{2}(3-m)=N$ If $n\gt N$, then $a_{n}=-2^{n}+3\lt-2^{\log_{2}(3-m)}+3=-(3-m)+3=m,$ $a_{n}\lt m$ $\{a_{n}\}$ diverges to negative infinity.
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