Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 13 - Section 13.3 - Arc Length and Curvature - 13.3 Exercise - Page 868: 24

Answer

$\dfrac{\sqrt{30}}{18}$

Work Step by Step

Given: $r(t)=\lt t^2, \ln t, t \ln t \gt$ To calculate the curvature of the curve we will have to use Theorem 10 such as: $\dfrac{|r'(t) \times r''(t)|}{|r'(t)|^3}$ Thus, $r'(t)=\lt 2t, \frac{1}{t}, \ln t +1 \gt$ and $r''(t)=\lt 2, -\frac{1}{t^2}, \frac{1}{t} \gt$ Now, $\dfrac{|r'(t) \times r''(t)|}{|r'(t)|^3}=\dfrac{|\frac{1}{t^2}(2+\ln t)i+2 \ln t j4/tk^ |}{[4t^2+\frac{1}{t^2}+ (\ln t+1)^2]^{3/2}}$ At point $(1,0,0)$ $\dfrac{|r'(t) \times r''(t)|}{|r'(t)|^3}=\dfrac{|2 i+0 j-4k |}{[6]^{3/2}}=\dfrac{\sqrt{30}}{18}$ Hence, $\dfrac{|r'(t) \times r''(t)|}{|r'(t)|^3}|_{(1,0,0)}=\dfrac{\sqrt{30}}{18}$
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