Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 1 - Limits and Their Properties - 1.2 Exercises - Page 56: 48

Answer

$$\lim_{x \to -4}(x^2+4x)=0$$For the proof by using the $\epsilon - \delta$ definition, see below.

Work Step by Step

$$\lim_{x \to -4} (x^2+4x)=(-4)^2+4(-4)=0$$Now, we want to prove this limit by using $\epsilon - \delta$ definition; that is, we must show that for each $\epsilon >0$, there exists a $\delta >0$ such that $|(x^2+4x)-0|< \epsilon$ whenever $|x-(-4)|< \delta$. Now, we have$$|x^2+4x|=|x||x+4|.$$For all $x$ in the interval $(-5,-3)$, $|x|<5$. So, letting $\delta$ be the minimum of $1$ and $\frac{\epsilon }{5}$, we conclude that$$|x-(-4)|< \delta =\min \{ 1, \frac{\epsilon }{5} \} \quad \Rightarrow \quad |(x^2+4x)-0|=|x||x+4|$$.
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