Calculus (3rd Edition)

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

Chapter 4 - Applications of the Derivative - 4.7 Newton's Method - Exercises - Page 220: 12

Answer

$5^{1/3}\approx1.709$

Work Step by Step

Given $$5^{1/3} $$ Let $$f(x) =x^3-5 $$ and suppose that $x_0=2$, then Since $$ x_{n+1}=x_{n}-\frac{f\left(x_{n}\right)}{f^{\prime}\left(x_{n}\right)}$$ Then \begin{align*} x_{n+1}&= x_n -\frac{x_n^{3}-5}{3x_n^2}\\ &=\frac{2x_n^{3}+5}{3x_n^2} \end{align*} Hence \begin{aligned} x_{1}&=\frac{2x_0^{3}+5}{3x_0^2}=\frac{2( 2)^{3}+5}{3(2) ^2} \approx 3.083 \\ x_{2}&=\frac{2x_1^{3}+5}{3x_1^2}= \frac{2( 3.083)^{3}+5}{3(3.083) ^2} \approx 2.230\\ x_{3}&=\frac{2x_2^{3}+5}{3x_2^2} =\frac{2(2.230)^{3}+5}{3(2.230) ^2} \approx1.821\\ x_{4}&=\frac{2x_3^{3}+5}{3x_3^2} =\frac{2(1.821)^{3}+5}{3(1.821) ^2} \approx1.716\\ x_{5}&=\frac{2x_4^{3}+5}{3x_4^2} =\frac{2(1.716)^{3}+5}{3(1.716) ^2} \approx1.709\\ x_{6}&=\frac{2x_5^{3}+5}{3x_5^2} =\frac{2(1.709)^{3}+5}{3(1.709) ^2} \approx1.709\\ \end{aligned} Since $x_5=x_6 $, then $5^{1/3}\approx1.709$ By calculator we get $$5^{1/3}=1.7099$$
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