Calculus: Early Transcendentals (2nd Edition)

Published by Pearson
ISBN 10: 0321947347
ISBN 13: 978-0-32194-734-5

Chapter 12 - Functions of Several Veriables - 12.6 Directional Derivatives and the Gradient - 12.6 Exercises - Page 925: 13

Answer

$$\nabla {\text{ }}f\left( {x,y} \right) = {e^{2xy}}\left\langle {2xy + 1,2{x^2}} \right\rangle {\text{ and }}\nabla {\text{ }}f\left( {1,0} \right) = \left\langle {1,2} \right\rangle $$

Work Step by Step

$$\eqalign{ & f\left( {x,y} \right) = x{e^{2xy}};\,\,\,\,\,\,P\left( {1,0} \right) \cr & {\text{compute the gradient of }}f\left( {x,y} \right),{\text{ }} \cr & {\text{first find the partial derivatives }}{f_x}\left( {x,y} \right){\text{ and }}{f_y}\left( {x,y} \right).{\text{ then}} \cr & {f_x}\left( {x,y} \right) = \frac{\partial }{{\partial x}}\left[ {x{e^{2xy}}} \right] \cr & {\text{treat }}y{\text{ as a constant}} \cr & {f_x}\left( {x,y} \right) = x\frac{\partial }{{\partial x}}\left[ {{e^{2xy}}} \right] + {e^{2xy}}\frac{\partial }{{\partial x}}\left[ x \right] \cr & {f_x}\left( {x,y} \right) = x\left( {2y{e^{2xy}}} \right) + {e^{2xy}}\left( 1 \right) \cr & {f_x}\left( {x,y} \right) = 2xy{e^{2xy}} + {e^{2xy}} \cr & and \cr & {f_y}\left( {x,y} \right) = \frac{\partial }{{\partial y}}\left[ {x{e^{2xy}}} \right] \cr & {\text{treat }}x{\text{ as a constant}} \cr & {f_y}\left( {x,y} \right) = 2{x^2}{e^{2xy}} \cr & {\text{using }}\nabla {\text{ }}f\left( {x,y} \right) = {f_x}\left( {x,y} \right){\bf{i}} + {f_y}\left( {x,y} \right){\bf{j}}.{\text{ then substituting the partial}} \cr & {\text{derivatives}} \cr & \nabla {\text{ }}f\left( {x,y} \right) = \left( {2xy{e^{2xy}} + {e^{2xy}}} \right){\bf{i}} + \left( {2{x^2}{e^{2xy}}} \right){\bf{j}} \cr & or \cr & \nabla {\text{ }}f\left( {x,y} \right) = \left\langle {2xy{e^{2xy}} + {e^{2xy}},2{x^2}{e^{2xy}}} \right\rangle \cr & \nabla {\text{ }}f\left( {x,y} \right) = {e^{2xy}}\left\langle {2xy + 1,2{x^2}} \right\rangle \cr & {\text{evaluate }}\nabla {\text{ }}f\left( {x,y} \right){\text{ at the point }}P \cr & \nabla {\text{ }}f\left( {1,0} \right) = {e^{2xy}}\left\langle {2xy + 1,2{x^2}} \right\rangle \cr & \nabla {\text{ }}f\left( {1,0} \right) = {e^{2\left( 1 \right)\left( 0 \right)}}\left\langle {2\left( 1 \right)\left( 0 \right) + 1,2{{\left( 1 \right)}^2}} \right\rangle \cr & \nabla {\text{ }}f\left( {1,0} \right) = \left\langle {1,2} \right\rangle \cr & \cr & \nabla {\text{ }}f\left( {x,y} \right) = {e^{2xy}}\left\langle {2xy + 1,2{x^2}} \right\rangle {\text{ and }}\nabla {\text{ }}f\left( {1,0} \right) = \left\langle {1,2} \right\rangle \cr} $$
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