Calculus (3rd Edition)

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

Chapter 11 - Infinite Series - Chapter Review Exercises - Page 593: 119

Answer

$f\left( x \right) = \dfrac{1}{{1 - 2x}} = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{2^n}}}{{{5^{n + 1}}}}{\left( {x + 2} \right)^n}$

Work Step by Step

Write $\dfrac{1}{{1 - 2x}} = \dfrac{1}{{1 - 2\left( {x + 2} \right) + 4}} = \dfrac{1}{{5 - 2\left( {x + 2} \right)}} = \dfrac{1}{{5\left( {1 - \dfrac{2}{5}\left( {x + 2} \right)} \right)}} = \dfrac{1}{5}\cdot\dfrac{1}{{1 - \dfrac{2}{5}\left( {x + 2} \right)}}$ So, $\dfrac{1}{{1 - 2x}} = \dfrac{1}{5}\cdot\dfrac{1}{{1 - \dfrac{2}{5}\left( {x + 2} \right)}}$. From Table 2 of Section 11.7, we know that $\dfrac{1}{{1 - x}} = \mathop \sum \limits_{n = 0}^\infty {x^n}$, ${\ \ \ }$ converges for $\left| x \right| \lt 1$ Substituting $\dfrac{2}{5}\left( {x + 2} \right)$ for $x$ in the series above, we obtain $\dfrac{1}{{1 - 2x}} = \dfrac{1}{5}\cdot\dfrac{1}{{1 - \dfrac{2}{5}\left( {x + 2} \right)}} = \dfrac{1}{5}\mathop \sum \limits_{n = 0}^\infty {\left( {\dfrac{2}{5}\left( {x + 2} \right)} \right)^n}$ $\dfrac{1}{{1 - 2x}} = \dfrac{1}{5}\mathop \sum \limits_{n = 0}^\infty \dfrac{{{2^n}}}{{{5^n}}}{\left( {x + 2} \right)^n} = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{2^n}}}{{{5^{n + 1}}}}{\left( {x + 2} \right)^n}$ Thus, the Taylor series centered at $c=-2$ is $f\left( x \right) = \dfrac{1}{{1 - 2x}} = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{2^n}}}{{{5^{n + 1}}}}{\left( {x + 2} \right)^n}$.
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