Calculus (3rd Edition)

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

Chapter 11 - Infinite Series - 11.7 Taylor Series - Exercises - Page 589: 75

Answer

This shows the first three terms of the Maclaurin series: ${{\rm{e}}^{{x^{20}}}} = 1 + {x^{20}} + \dfrac{{{x^{40}}}}{{2!}} + \cdot \cdot \cdot $ We show that the coefficients of the series are zero for $1 \le k \le 19$. Therefore, we conclude that ${f^{\left( k \right)}}\left( 0 \right) = 0$ for $1 \le k \le 19$.

Work Step by Step

From Table 2, we get ${{\rm{e}}^x} = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{x^n}}}{{n!}} = 1 + x + \dfrac{{{x^2}}}{{2!}} + \dfrac{{{x^3}}}{{3!}} + \dfrac{{{x^4}}}{{4!}} + \cdot\cdot\cdot$, ${\ \ \ }$ converges for all $x$. Substituting ${x^{20}}$ for $x$ in the series above gives ${{\rm{e}}^{{x^{20}}}} = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{x^{20n}}}}{{n!}} = 1 + {x^{20}} + \dfrac{{{x^{40}}}}{{2!}} + \dfrac{{{x^{60}}}}{{3!}} + \dfrac{{{x^{80}}}}{{4!}} + \cdot\cdot\cdot$, ${\ \ \ }$ converges for all $x$. According to Maclaurin series: $f\left( x \right) = \mathop \sum \limits_{n = 0}^\infty \dfrac{{{f^{\left( n \right)}}\left( 0 \right)}}{{n!}}{x^n}$. For the first three terms, we get $\dfrac{{{f^{\left( 0 \right)}}\left( 0 \right)}}{{0!}} = 1$ ${\ \ \ }$ $ \Rightarrow $ ${\ \ \ }$ ${f^{\left( 0 \right)}}\left( 0 \right) = 1$ $\dfrac{{{f^{\left( {20} \right)}}\left( 0 \right)}}{{20!}} = 1$ ${\ \ \ }$ $ \Rightarrow $ ${\ \ \ }$ ${f^{\left( {20} \right)}}\left( 0 \right) = 20!$ $\dfrac{{{f^{\left( {40} \right)}}\left( 0 \right)}}{{40!}} = \dfrac{1}{{2!}}$ ${\ \ \ }$ $ \Rightarrow $ ${\ \ \ }$ ${f^{\left( {40} \right)}}\left( 0 \right) = \dfrac{{40!}}{{2!}}$ Notice that the coefficients of the series are zero for $1 \le k \le 19$. Therefore, we conclude that ${f^{\left( k \right)}}\left( 0 \right) = 0$ for $1 \le k \le 19$.
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