Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 13 - Functions of Several Variables - 13.8 Exercises - Page 942: 14

Answer

$${\text{Relative minimum at the point }}\left( {8,16,74} \right)$$

Work Step by Step

$$\eqalign{ & z = - 5{x^2} + 4xy - {y^2} + 16x + 10 \cr & {\text{Let }}z = f\left( {x,y} \right) \cr & f\left( {x,y} \right) = - 5{x^2} + 4xy - {y^2} + 16x + 10 \cr & {\text{Calculate the first partial derivatives of }}f\left( {x,y} \right) \cr & {f_x}\left( {x,y} \right) = \frac{\partial }{{\partial x}}\left[ { - 5{x^2} + 4xy - {y^2} + 16x + 10} \right] \cr & {f_x}\left( {x,y} \right) = - 10x + 4y + 16 \cr & and \cr & {f_y}\left( {x,y} \right) = \frac{\partial }{{\partial y}}\left[ { - 5{x^2} + 4xy - {y^2} + 16x + 10} \right] \cr & {f_y}\left( {x,y} \right) = 4x - 2y \cr & {\text{Setting both first partial derivatives equal to zero, we have}} \cr & {f_x}\left( {x,y} \right) = 0,{\text{ }}{f_y}\left( {x,y} \right) = 0 \cr & - 10x + 4y + 16 = 0,{\text{ }}4x - 2y = 0 \cr & {\text{Solving the system of equations, we obtain}} \cr & x = 8,{\text{ }}y = 16 \cr & {\text{The critical point is }}\left( {8,16} \right) \cr & {\text{Find the second partial derivatives of }}f\left( {x,y} \right){\text{ and }}{f_{xy}}\left( {x,y} \right) \cr & {f_{xx}}\left( {x,y} \right) = \frac{\partial }{{\partial x}}\left[ { - 10x + 4y + 16} \right] = - 10 \cr & {f_{yy}}\left( {x,y} \right) = \frac{\partial }{{\partial y}}\left[ {4x - 2y} \right] = - 2 \cr & {f_{xy}}\left( {x,y} \right) = \frac{\partial }{{\partial y}}\left[ { - 10x + 4y + 16} \right] = 4 \cr & {\text{By the second partials test}} \cr & d\left( {x,y} \right) = {f_{xx}}\left( {a,b} \right){f_{yy}}\left( {a,b} \right) - {\left[ {{f_{xy}}\left( {x,y} \right)} \right]^2} \cr & {\text{Evaluate }}d\left( {x,y} \right){\text{ at the critical point }}\left( {8,16} \right) \cr & d\left( {8,16} \right) = {f_{xx}}\left( {8,16} \right){f_{yy}}\left( {8,16} \right) - {\left[ {{f_{xy}}\left( {8,16} \right)} \right]^2} \cr & d\left( {8,16} \right) = \left( { - 10} \right)\left( { - 2} \right) - {\left[ 4 \right]^2} \cr & d\left( {8,16} \right) = 4 \cr & d > 0,{\text{ and }}{f_{xx}}\left( {8,16} \right) = 4 > 0 \cr & {\text{then}} \cr & f\left( {x,y} \right){\text{ has a relative minimum at }}\left( {8,16,f\left( {8,16} \right)} \right) \cr & f\left( {x,y} \right) = - 5{x^2} + 4xy - {y^2} + 16x + 10 \cr & f\left( {8,16} \right) = - 5{\left( 8 \right)^2} + 4\left( 8 \right)\left( {16} \right) - {\left( {16} \right)^2} + 16\left( 8 \right) + 10 \cr & f\left( {3, - 4} \right) = 74 \cr & {\text{Relative minimum at the point }}\left( {8,16,74} \right) \cr} $$
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