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: 77

Answer

No, it does not converge at $x=2$.

Work Step by Step

By Theorem 3: ${\left( {1 + x} \right)^a} = 1 + \dfrac{a}{{1!}}x + \dfrac{{a\left( {a - 1} \right)}}{{2!}}{x^2} + \dfrac{{a\left( {a - 1} \right)\left( {a - 2} \right)}}{{3!}}{x^3} + \cdot\cdot\cdot + \left( {\begin{array}{*{20}{c}} a\\ n \end{array}} \right){x^n} + \cdot\cdot\cdot$ converges for $\left| x \right| \lt 1$. Setting $a = \dfrac{3}{4}$ in the series above gives $f\left( x \right) = {\left( {1 + x} \right)^{3/4}} = 1 + \dfrac{{\dfrac{3}{4}}}{{1!}}x + \dfrac{{\dfrac{3}{4}\left( { - \dfrac{1}{4}} \right)}}{{2!}}{x^2} + \dfrac{{\dfrac{3}{4}\left( { - \dfrac{1}{4}} \right)\left( { - \dfrac{5}{4}} \right)}}{{3!}}{x^3} + \cdot\cdot\cdot + \left( {\begin{array}{*{20}{c}} {3/4}\\ n \end{array}} \right){x^n} + \cdot\cdot\cdot$ Since the series converges for $\left| x \right| \lt 1$, it does not converge at $x=2$. At $x=2$, the terms are increasing and the series approaches infinity, therefore it does not converge. Moreover, since the term does not converges to zero. By the $n$th Term Divergence Test, the series diverges at $x=2$.
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