## University Calculus: Early Transcendentals (3rd Edition)

Published by Pearson

# Chapter 2 - Section 2.6 - Limits Involving Infinity; Asymptotes of Graphs - Exercises - Page 109: 92

#### Answer

Prove that for every positive real number $B$, there exists a corresponding $\delta\gt0$ such that for all $x$ $$0\lt|x+5|\lt\delta\Rightarrow f(x)\gt B$$

#### Work Step by Step

*The formal definition of infinite limits: $\lim_{x\to c}f(x)=\infty$ if for every positive real number $B$, there exists a corresponding number $\delta\gt0$ such that for all $x$ $$0\lt|x-c|\lt \delta\Rightarrow f(x)\gt B$$ $$\lim_{x\to-5}\frac{1}{(x+5)^2}=\infty$$ We need to prove here that for every positive real number $B$, there exists a corresponding $\delta\gt0$ such that for all $x$ $$0\lt|x+5|\lt\delta\Rightarrow f(x)\gt B$$ - Examine the inequality: $$f(x)\gt B$$ $$\frac{1}{(x+5)^2}\gt B$$ $$(x+5)^2\lt \frac{1}{B}$$ $$|x+5|\lt\frac{1}{\sqrt B}$$ - So if we set $\delta=\frac{1}{\sqrt B}$ here, that would make $0\lt |x|\lt\frac{1}{ \sqrt B}$, then for all $x$, we would have $f(x)\gt B$. The limit has been proved then.

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