Solve Multivariable Extrema for f(x) = x^2 + 4xy + y^2 + 6x + 8

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Homework Help Overview

The discussion revolves around finding the extrema of the function f(x, y) = x^2 + 4xy + y^2 + 6x + 8, focusing on identifying minimum, maximum, or saddle points using multivariable calculus techniques.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of the determinant of the Hessian matrix and its implications for determining extrema. There is a suggestion to consider a change of coordinates to address symmetry in the function. Questions are raised about the correctness of the determinant calculation.

Discussion Status

The discussion is active, with participants questioning the validity of the determinant being zero and exploring alternative methods. Some guidance has been offered regarding changing coordinates, but there is no consensus on the next steps.

Contextual Notes

Participants note the absence of constraints for using Lagrange multipliers, which adds complexity to the problem. There is also a recognition of the potential breakdown of certain methods due to the determinant issue.

thshen34
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Homework Statement


f = x^2 + 4xy + y^2 + 6x + 8
Find minimum, maximum, or saddle point

Homework Equations


A = [f_xx, f_xy; f_yx, f_yy]

The Attempt at a Solution



Found the determinant to be zero ( i got [2 ,4; 4, 2] ) so i can't use the eigenvalues to determine the extrema.

now what do i do? If I am to use lagrange multipliers, how do i do that without a constraint?
 
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Yes, that method breaks down here. Try a change of coordinates to break the symmetry between the second order x and y terms.
 
How is the determinant 0?

EDIT: Sorry, I forgot I can't give out full solutions.

Make sure you are finding the determinant correctly.
 
Last edited:
Karnage1993 said:
How is the determinant 0?
Good point - I failed to check that claim.
 

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