Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 1 - Limits and Continuity - 1.3 Limits At Infinity; End Behavior Of A Function - Exercises Set 1.3 - Page 79: 37

Answer

The limit does not exist.

Work Step by Step

$\lim\limits_{x\to \infty} \ln(\frac{2}{x^{2}})$ We know that $\lim\limits_{x \to \infty} \frac{1}{x} = 0$. Hence, $\lim\limits_{x \to \infty} \frac{1}{x^{2}} = 0$. Hence, $\lim\limits_{x \to \infty} \frac{2}{x^{2}} = 0$. Now, substitute $\frac{2}{x^{2}} = t$. Thus, notice that when x tends to infinity, $t$ tends to 0. Hence, this is the modified limit: $\lim\limits_{t \to 0} \ln t$ We know that when $t = 0$, $\ln t$ does not exist ......($\ln 0$ does not exist) But, here, $t$ is tending to 0, but not equal to 0. Look at the graph of $\ln t$. It approaches negative infinity as $t$ approaches 0 from the right hand side (positive $t$). However, when we say $\lim\limits_{t \to 0} \ln t$, we imply that we approach $0$ from both sides: $0+$ and $0-$. In this case, $\lim\limits_{t \to 0-} \ln t$ does not exist (look at the graph attached). Thus, in this case, the limit simply does not exist.
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