Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 15 - Differentiation in Several Variables - 15.6 The Chain Rule - Exercises - Page 808: 11

Answer

$\dfrac{\partial f(-1,-1)}{\partial u}|_{x=0,y=0,z=1}=1$ and $\dfrac{\partial f(-1,-1)}{\partial v}|_{x=0,y=0,z=1}=-2$

Work Step by Step

We have $\dfrac{\partial f}{\partial x}=3x^2; \dfrac{\partial f}{\partial y}=z^2; \dfrac{\partial f}{\partial z}=2yz; \dfrac{\partial x}{\partial u}=2u; \dfrac{\partial y}{\partial u}=1$ and $\dfrac{\partial z}{\partial u}=v; \dfrac{\partial x}{\partial v}=1; \dfrac{\partial y}{\partial v}=2v; \dfrac{\partial z}{\partial v}=u$ We apply the chain rule to obtain: $\dfrac{\partial f}{\partial u}=\dfrac{\partial f}{\partial x} \dfrac{\partial x}{\partial u}+\dfrac{\partial f}{\partial y} \dfrac{\partial y}{\partial u}+\dfrac{\partial f}{\partial z} \dfrac{\partial z}{\partial u} \\=6ux^2+z^2+2vyz$ $\dfrac{\partial f}{\partial v}=\dfrac{\partial f}{\partial x} \dfrac{\partial x}{\partial v}+\dfrac{\partial f}{\partial y} \dfrac{\partial y}{\partial v}+\dfrac{\partial f}{\partial z} \dfrac{\partial z}{\partial v} \\=3x^2+2vz^2+2uyz$ Now, $\dfrac{\partial f(-1,-1)}{\partial u}|_{x=0,y=0,z=1}=6(-1)(0)+(1)^2+2(-1)(0)(1)=1$ and $\dfrac{\partial f(-1,-1)}{\partial v}|_{x=0,y=0,z=1}=3(0)+2(-1)(1)+2(-1)(0)(1)=-2$
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