Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 2 - Limits - 2.9 The Formal Definition of a Limit - Exercises - Page 93: 21

Answer

$$ \lim_{x\to 1}x^3= 1$$

Work Step by Step

Let $\epsilon\gt0$ be given. We bound $\left|x^{3}-1\right|$ by factoring the difference of cubes: $$ \left|x^{3}-1\right|=\left|\left(x^{2}+x+1\right)(x-1)\right|=|x-1|\left|x^{2}+x+1\right| $$ Let $\delta=\min (1, \xi),$ and assume $|x-1|\lt\delta .$ Since $\delta\lt1,0\lt x\lt 2 .$ Since $x^{2}+x+1$ increases as $x$ increases for $x\gt 0, x^{2}+x+1\lt 7$ for $0\lt x\lt 2,$ and so $$ \left|x^{3}-1\right|=|x-1|\left|x^{2}+x+1\right|\lt 7|x-1|\lt 7 \frac{\epsilon}{7}=\epsilon $$ Hence $$ \lim_{x\to 1}x^3= 1$$
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