Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 7: Transcendental Functions - Practice Exercises - Page 439: 14

Answer

$$\frac{{dy}}{{dx}} = {\left( {\ln x} \right)^{x/2}}\left( {\frac{1}{{\ln x}} + \ln \left( {\ln x} \right)} \right)$$

Work Step by Step

$$\eqalign{ & y = 2{\left( {\ln x} \right)^{x/2}} \cr & \frac{y}{2} = {\left( {\ln x} \right)^{x/2}} \cr & {\text{Take the natural logarithm on both sides}} \cr & \ln \frac{y}{2} = \ln {\left( {\ln x} \right)^{x/2}} \cr & {\text{use the power property for logarithms}} \cr & \ln \frac{y}{2} = \ln {\left( {\ln x} \right)^{x/2}} \cr & {\text{differentiate both sides with respect to }}x \cr & \frac{1}{y}\frac{{dy}}{{dx}} = \frac{d}{{dx}}\left( {\frac{x}{2}\left( {\ln \left( {\ln x} \right)} \right)} \right) \cr & {\text{use the product rule for derivatives}} \cr & \frac{1}{y}\frac{{dy}}{{dx}} = \frac{x}{2}\frac{d}{{dx}}\left( {\ln \left( {\ln x} \right)} \right) + \ln \left( {\ln x} \right)\frac{d}{{dx}}\left( {\frac{x}{2}} \right) \cr & \frac{1}{y}\frac{{dy}}{{dx}} = \frac{x}{2}\left( {\frac{1}{{x\ln x}}} \right) + \ln \left( {\ln x} \right)\left( {\frac{1}{2}} \right) \cr & \frac{1}{y}\frac{{dy}}{{dx}} = \frac{1}{{2\ln x}} + \frac{{\ln \left( {\ln x} \right)}}{2} \cr & {\text{solve the equation for }}\frac{{dy}}{{dx}} \cr & \frac{{dy}}{{dx}} = y\left( {\frac{1}{{2\ln x}} + \frac{{\ln \left( {\ln x} \right)}}{2}} \right) \cr & {\text{replace }}y = 2{\left( {\ln x} \right)^{x/2}} \cr & \frac{{dy}}{{dx}} = 2{\left( {\ln x} \right)^{x/2}}\left( {\frac{1}{{2\ln x}} + \frac{{\ln \left( {\ln x} \right)}}{2}} \right) \cr & \frac{{dy}}{{dx}} = {\left( {\ln x} \right)^{x/2}}\left( {\frac{1}{{\ln x}} + \ln \left( {\ln x} \right)} \right) \cr} $$
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