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91Ó°ÊÓ

Use Theorem 12.32 to find the indicated derivatives in Exercises

21–26. Express your answers as functions of a single variable

w=lnx2yz+lnzxy-lnxy2,P(3,5,8),v→=13i+21j+34k

Short Answer

Expert verified

The direction derivative of the given function at P(3,5,8)in the direction v→

role="math" localid="1653917061156" Dv→w(x,y,z)=0

Step by step solution

01

Given information

The function w=lnx2yz+lnzxy-lnxy2…….(1)

And the given vector

v→=13i+21j+34k

02

The objective is to find the direct derivative of the given function atP(3,5,8)  in the direction v→

The direction derivative of a function f(x,y,z)in the direction of the unit vector u→=ai+bj+cj

Du(x,y,z)=fx(x,y,z)a+fy(x,y,z)b+fz(x,y,z)c=∇f(x,y,z)·u→

Rewrite function (1)as follows:

w=ln(x2yz)+ln(zxy)-ln(xy2)=ln(x2yz)(zxy)-ln(xy2)=ln(xy2)(y2x)=ln1=0Thus,∇w=0

Hence, the directional derivative of the given function at P(3,5,8)in the direction v→

Dv→w(x,y,z)=∇w(x,y,z)·v→=0·v→=0

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