Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 13 - Partial Derivatives - 13.5 The Chain Rule - Exercises Set 13.5 - Page 957: 10

Answer

$$\,\frac{{dw}}{{dt}} = \frac{{1 - 512{t^5}}}{{2t\sqrt {1 + \ln t - 512{t^5}\ln t} }} - \frac{{256{t^4}\ln t}}{{\sqrt {1 + \ln t - 512{t^5}\ln t} }} - \frac{{1024{t^4}\ln t}}{{\sqrt {1 + \ln t - 512{t^5}\ln t} }}$$

Work Step by Step

$$\eqalign{ & w = \sqrt {1 + x - 2y{z^4}x} ;\,\,\,\,\,x = \ln t,\,\,\,y = t,\,\,\,z = 4t \cr & {\text{Calculate the partial derivatives }}\frac{{\partial w}}{{\partial x}}{\text{,}}\,\,\frac{{\partial w}}{{\partial y}}{\text{ and }}\frac{{\partial w}}{{\partial z}} \cr & \,\,\,\,\frac{{\partial w}}{{\partial x}} = \frac{\partial }{{\partial x}}\left( {\sqrt {1 + x - 2y{z^4}x} } \right) \cr & \,\,\,\,{\text{treat }}y{\text{ and }}z{\text{ as constants}} \cr & \,\,\,\,\frac{{\partial w}}{{\partial x}} = \frac{1}{2}{\left( {1 + x - 2y{z^4}x} \right)^{ - 1/2}}\left( {1 - 2y{z^4}} \right) \cr & \,\,\,\,\frac{{\partial w}}{{\partial x}} = \frac{{1 - 2y{z^4}}}{{2\sqrt {1 + x - 2y{z^4}x} }} \cr & \cr & \,\,\,\,\frac{{\partial w}}{{\partial y}} = \frac{\partial }{{\partial y}}\left( {\sqrt {1 + x - 2y{z^4}x} } \right) \cr & \,\,\,\,{\text{treat }}x{\text{ and }}z{\text{ as constants}} \cr & \,\,\,\,\frac{{\partial w}}{{\partial y}} = \frac{1}{2}{\left( {1 + x - 2y{z^4}x} \right)^{ - 1/2}}\left( { - 2x{z^4}} \right) \cr & \,\,\,\,\frac{{\partial w}}{{\partial y}} = - \frac{{x{z^4}}}{{\sqrt {1 + x - 2y{z^4}x} }} \cr & \cr & \,\,\,\,\frac{{\partial w}}{{\partial z}} = \frac{\partial }{{\partial z}}\left( {\sqrt {1 + x - 2y{z^4}x} } \right) \cr & \,\,\,\,{\text{treat }}x{\text{ and }}y{\text{ as constants}} \cr & \,\,\,\,\frac{{\partial w}}{{\partial z}} = \frac{1}{2}{\left( {1 + x - 2y{z^4}x} \right)^{ - 1/2}}\left( { - 8xy{z^3}} \right) \cr & \,\,\,\,\frac{{\partial w}}{{\partial z}} = - \frac{{4xy{z^3}}}{{\sqrt {1 + x - 2y{z^4}x} }} \cr & \cr & {\text{Calculate the ordinary derivatives }}\frac{{dx}}{{dt}}{\text{,}}\,\,{\text{ }}\frac{{dy}}{{dt}}{\text{ and }}\frac{{dz}}{{dt}} \cr & \,\,\,\,\frac{{dx}}{{dt}} = \frac{d}{{dt}}\left( {\ln t} \right) = \frac{1}{t} \cr & \,\,\,\,\frac{{dy}}{{dt}} = \frac{d}{{dt}}\left( t \right) = 1 \cr & \,\,\,\,\frac{{dz}}{{dt}} = \frac{d}{{dt}}\left( {4t} \right) = 4 \cr & \cr & {\text{Use the Theorem 13}}{\text{.5}}{\text{.1}}\,\,\left( {{\text{chain rule for derivatives}}} \right){\text{ see page 950}} \cr & \,\,\,\,\frac{{dw}}{{dt}} = \frac{{\partial w}}{{\partial x}}\frac{{dx}}{{dt}} + \frac{{\partial w}}{{\partial y}}\frac{{dy}}{{dt}} + \frac{{\partial w}}{{\partial y}}\frac{{dz}}{{dt}} \cr & \,\,\,\,\,{\text{Substitute the derivatives}} \cr & \,\,\,\,\frac{{dw}}{{dt}} = \left( {\frac{{1 - 2y{z^4}}}{{2\sqrt {1 + x - 2y{z^4}x} }}} \right)\left( {\frac{1}{t}} \right) + \left( { - \frac{{x{z^4}}}{{\sqrt {1 + x - 2y{z^4}x} }}} \right) + \left( { - \frac{{4xy{z^3}}}{{\sqrt {1 + x - 2y{z^4}x} }}} \right)\left( 4 \right) \cr & \,\,\,\,\frac{{dw}}{{dt}} = \frac{{1 - 2y{z^4}}}{{2t\sqrt {1 + x - 2y{z^4}x} }} - \frac{{x{z^4}}}{{\sqrt {1 + x - 2y{z^4}x} }} - \frac{{16xy{z^3}}}{{\sqrt {1 + x - 2y{z^4}x} }} \cr & \,\,\,\,{\text{Where }}\,x = \ln t,\,\,\,y = t,\,\,\,z = 4t \cr & \,\,\,\,\frac{{dw}}{{dt}} = \frac{{1 - 2\left( t \right){{\left( {4t} \right)}^4}}}{{2t\sqrt {1 + \ln t - 2t{{\left( {4t} \right)}^4}\ln t} }} - \frac{{{{\left( {4t} \right)}^4}\ln t}}{{\sqrt {1 + \ln t - 2t{{\left( {4t} \right)}^4}\ln t} }} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \frac{{16t{{\left( {4t} \right)}^3}\ln t}}{{\sqrt {1 + \ln t - 2t{{\left( {4t} \right)}^4}\ln t} }} \cr & \,\frac{{dw}}{{dt}} = \frac{{1 - 512{t^5}}}{{2t\sqrt {1 + \ln t - 512{t^5}\ln t} }} - \frac{{256{t^4}\ln t}}{{\sqrt {1 + \ln t - 512{t^5}\ln t} }} - \frac{{1024{t^4}\ln t}}{{\sqrt {1 + \ln t - 512{t^5}\ln t} }} \cr} $$
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