Solve Schaum's Vector Analysis Prob 4.65: Find Constants a,b,c

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SUMMARY

The discussion centers on solving Schaum's Vector Analysis Problem 4.65, which requires finding constants a, b, and c such that the directional derivative of the function phi = axy² + byz + cz²x³ at the point (1, 2, -1) achieves a maximum magnitude of 64 in the direction parallel to the z-axis. The solution involves calculating the gradient of phi at the specified point, yielding the equations (2b - 2c) = 64 and (4a + 3c) = (4a - b) = 0. By solving these equations, the values of a, b, and c can be determined definitively.

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Homework Statement


Schaum's vector analysis prob 4.65
Find the values of constants a,b,c so that directional derivative of phi=axy^2+byz+cz^2x^3 at (1,2,-1) has a maximum of magnitude 64 in a direction parallel to the z axis.


Homework Equations





The Attempt at a Solution


Directional derivative is maximum along grad(phi). After taking grad(phi) at (1,2,-1) I got
(4a+3c)i+(4a-b)j+(2b-2c)k
I don't know how to make use of the 'direction parallel to z axis' condition.
 
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If grad(phi) is maximum in Z-axis, it may be zero in x and y-axis. Then
(2b - 2c) = 64.
(4a+3c) = (4a-b) = 0.
Solve these equations to find a, b and c.
 
Got it! Thanks.
 

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