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: 120

Answer

$f\left( x \right) = \dfrac{1}{{{{\left( {1 - 2x} \right)}^2}}} = \mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} { - 2}\\ n \end{array}} \right){\left( { - 1} \right)^n}\dfrac{{{2^n}}}{{{5^{n + 2}}}}{\left( {x + 2} \right)^n}$

Work Step by Step

Write $\dfrac{1}{{{{\left( {1 - 2x} \right)}^2}}} = \dfrac{1}{{{{\left( {1 - 2\left( {x + 2} \right) + 4} \right)}^2}}} = \dfrac{1}{{{{\left( {5 - 2\left( {x + 2} \right)} \right)}^2}}} = \dfrac{1}{{{5^2}{{\left( {1 - \dfrac{2}{5}\left( {x + 2} \right)} \right)}^2}}} = \dfrac{1}{{{5^2}}}{\left( {1 - \dfrac{2}{5}\left( {x + 2} \right)} \right)^{ - 2}}$ So, $\dfrac{1}{{{{\left( {1 - 2x} \right)}^2}}} = \dfrac{1}{{{5^2}}}{\left( {1 - \dfrac{2}{5}\left( {x + 2} \right)} \right)^{ - 2}}$. From Table 2 of Section 11.7, we know that ${\left( {1 + x} \right)^a} = \mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} a\\ n \end{array}} \right){x^n}$, converges for $\left| x \right| \lt 1$ Setting $a=-2$ and substituting $ - \dfrac{2}{5}\left( {x + 2} \right)$ for $x$ in the series above, we obtain $\dfrac{1}{{{{\left( {1 - 2x} \right)}^2}}} = \dfrac{1}{{{5^2}}}{\left( {1 - \dfrac{2}{5}\left( {x + 2} \right)} \right)^{ - 2}} = \dfrac{1}{{{5^2}}}\mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} { - 2}\\ n \end{array}} \right){\left( { - \dfrac{2}{5}\left( {x + 2} \right)} \right)^n}$ $ = \dfrac{1}{{{5^2}}}\mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} { - 2}\\ n \end{array}} \right){\left( { - 1} \right)^n}\dfrac{{{2^n}}}{{{5^n}}}{\left( {x + 2} \right)^n}$ $ = \mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} { - 2}\\ n \end{array}} \right){\left( { - 1} \right)^n}\dfrac{{{2^n}}}{{{5^{n + 2}}}}{\left( {x + 2} \right)^n}$ Thus, the Taylor series centered at $c=-2$ is $f\left( x \right) = \dfrac{1}{{{{\left( {1 - 2x} \right)}^2}}} = \mathop \sum \limits_{n = 0}^\infty \left( {\begin{array}{*{20}{c}} { - 2}\\ n \end{array}} \right){\left( { - 1} \right)^n}\dfrac{{{2^n}}}{{{5^{n + 2}}}}{\left( {x + 2} \right)^n}$
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