University Calculus: Early Transcendentals (3rd Edition)

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

Chapter 2 - Section 2.3 - The Precise Definition of a Limit - Exercises - Page 77: 53

Answer

The statement is not sufficient to define the limit of $f(x)$ as $x$ approaches $c$. See the counter example below.

Work Step by Step

"The number $L$ is the limit of $f(x)$ as $x$ approaches $c$ if $f(x)$ gets closer to $L$ as $x$ approaches $c$." This statement is not sufficient to define the limit of $f(x)$ as $x$ approaches $c$. Take this example: $$f(x)= x^4-9$$ As $x$ approaches $0$, $f(x)$ actually does not just come closer to $-9$. It also comes closer to $-10, -11, -12, -13$ and so so on. Yet we know that $\lim_{x\to0}(x^4-9)=-9$, and there is only one value to that. Saying $f(x)$ gets closer to $L$ as $x$ approaches $c$, hence, is not enough to find $\lim_{x\to c}f(x)$, as in the case of $\lim_{x\to0}(x^4-9)$ above, there are too many $L's$ that $f(x)$ "gets closer to" as $x$ approaches $c$.
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