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

In summary, The conversation discusses finding the minimum, maximum, or saddle point of a function and using the determinant to determine extrema. The method of using eigenvalues breaks down in this case and a change of coordinates is suggested to break the symmetry between the second order x and y terms. The accuracy of the determinant calculation is also questioned.
  • #1
thshen34
13
0

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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  • #2
Yes, that method breaks down here. Try a change of coordinates to break the symmetry between the second order x and y terms.
 
  • #3
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:
  • #4
Karnage1993 said:
How is the determinant 0?
Good point - I failed to check that claim.
 

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

1. What is the definition of a simple multivariable extrema?

A simple multivariable extrema is a point where the value of a multivariable function is either at its highest or lowest point.

2. How do you find the extrema of a simple multivariable function?

To find the extrema of a simple multivariable function, you must first take partial derivatives with respect to each variable and set them equal to zero. Then, solve the resulting system of equations to find the critical points. Finally, use the second derivative test or a graphical method to determine the nature of the critical point and whether it is a maximum or minimum.

3. What is the second derivative test for determining extrema?

The second derivative test involves taking the second derivative of a function at a critical point. If the second derivative is positive, the critical point is a minimum. If the second derivative is negative, the critical point is a maximum. If the second derivative is equal to zero, the test is inconclusive and further analysis is needed.

4. Can a simple multivariable function have more than one extrema?

Yes, a simple multivariable function can have multiple extrema. This can occur when there are multiple critical points or when the function has a saddle point, where it is neither a maximum nor a minimum.

5. How is the concept of extrema applied in real-world situations?

The concept of extrema is applied in various fields of science, such as economics, physics, and engineering. For example, in economics, extrema can be used to determine the maximum profit or minimum cost for a business. In physics, extrema can be used to find the maximum or minimum speed of an object in motion. In engineering, extrema can be used to optimize the design of a structure or system.

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