Multivariable Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 0-53849-787-4
ISBN 13: 978-0-53849-787-9

Chapter 14 - Partial Derivatives - 14.8 Exercises - Page 987: 18

Answer

Maximum: $f(2,1,0)=5$ and Minimum: $f(0,-1,0)=1$

Work Step by Step

Use Lagrange Multipliers Method: $\nabla f(x,y,z)=\lambda_1 \nabla g(x,y,z)+\lambda_2 \nabla h(x,y,z)$ This yields $\nabla f=\lt 2x,2y,2z \gt$ and $\lambda_1 \nabla g(x,y,z)+\lambda_2 \nabla h(x,y,z)= \lt \lambda_1,-\lambda_1+2\lambda_2y,-2\lambda_2 z \gt$ Using the constraint condition $x-y=1$we get, $x=2/3 ,y=-1/3$ If we use the above value of $x,y$ in second constraint $y^2-z^2=1$, we will get an invalid value. Thus, we have to consider $z=0$; after solving, we get $x=2,0$ and $y=1,-1$ Hence, Maximum: $f(2,1,0)=5$ and Minimum: $f(0,-1,0)=1$
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