Understanding Critical Points in Multivariable Functions

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Homework Help Overview

The discussion revolves around the function f(x,y,z)=(xy+yz+xz)/(1+x^2+y^2+z^2) and the exploration of its critical points, as well as the reasoning behind the absence of absolute maximum or minimum values. Participants are examining the implications of different cases related to the variables x, y, and z.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to analyze the function by considering three specific cases to understand its behavior. Some participants question how to bound the function from above and below, and whether it can take on arbitrarily large values. Others suggest exploring additional cases and using cylindrical coordinates for further insights.

Discussion Status

The discussion is ongoing, with participants providing hints and exploring various cases. There is no explicit consensus yet, but the conversation is generating productive lines of inquiry regarding the function's properties.

Contextual Notes

Participants are working within the constraints of a homework assignment, which may limit the information available for analysis. The original poster expresses uncertainty about how to approach one of the cases and the reasoning behind the function's lack of absolute extrema.

erica1451
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Homework Statement


f(x,y,z)=(xy+yz+xz)/(1+x^2+y^2+z^2)
Explain why f has no absolute maximum or minimum. How about critical points?


Homework Equations


Hint: it is simplest to make 3 cases: a) x+y+z does not =0 b) x+y+z=0 c) x=y=z=0


The Attempt at a Solution


I did cases b and c, but I'm not sure how to go about doing a. Also, I'm not sure how to explain why the function does not have an absolute max or min.
 
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Can anyone help?
 
hmmm, simplifying things,

[tex]f(x,y,z)=\frac{1}{2}\cdot\left[\frac{(x+y+z)^2+1}{1+x^2+y^2+z^2}-1\right][/tex]
how can you bound f(x,y,z) from below? what about from above? can you make f arbitrarily big?
can you make f(x,y,z) arbitrary close to some values? try some additional cases, suppose x=y=z not equaling zero?

edit: additional hint: cylindrical coordinate.
 
Last edited:
Thank you!
 

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