Max/Min using Lagrange Multipliers: F(x,y) = x^2 + y^2 ; xy = 1

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In summary, to find the maximum or minimum values of the function F(x,y) = x^2 + y^2 subject to the constraint xy = 1, we use the method of Lagrange multipliers. By setting up the equations F_x = L*G_x and F_y = L*G_y and the constraint equation G(x,y) = 1, we can solve for the values of x and y in terms of L. However, the step x = L*y/2 and y = L*x/2 does not lead to the solution x = L^2/4 and y = L^2/4.
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tnutty
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Homework Statement



Find max/min using L.M of the function :

F(x,y) = x^2 + y^2 ; xy = 1

let G(x,y) = xy - 1

F_x = 2x
F_y = 2y
G_x = y
G_y = x

F_x = L*G_x
F_y = L*G_y
G(x,y) = 1


1) 2x = L * y
2) 2y = L * x
3 ) xy = 1

Now I need to solve those equations.

so x = LY/2 ; y = LX/2

Then if I plug it in I get X = L^2/4 and similar for Y
Not sure how to approach that step
 
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  • #2
x=Ly/2 and y=Lx/2 doesn't tell you x=L^2/4. Try that again.
 

1. What are Lagrange multipliers?

Lagrange multipliers are a mathematical tool used to optimize a function subject to equality constraints. They are named after Joseph-Louis Lagrange, who first described them in the late 18th century.

2. How do Lagrange multipliers work?

Lagrange multipliers work by introducing a new variable, called the multiplier, to the original function. This allows the optimization problem to be reformulated into a simpler form, making it easier to solve.

3. What types of problems can be solved using Lagrange multipliers?

Lagrange multipliers are commonly used in optimization problems with equality constraints, such as finding the maximum or minimum value of a function subject to certain constraints. They can also be used in physics and engineering to solve problems involving multiple variables and constraints.

4. Are there any limitations to using Lagrange multipliers?

While Lagrange multipliers are a powerful tool for solving optimization problems, they do have some limitations. They may not be effective for problems with inequality constraints, and they can be computationally expensive for large, complex problems.

5. How are Lagrange multipliers related to the method of Lagrange multipliers?

The method of Lagrange multipliers is a specific algorithm used to solve optimization problems using Lagrange multipliers. It involves setting up a system of equations and solving for the optimal values of the original function and the multiplier. The method is often used in conjunction with other optimization techniques to find the best solution to a problem.

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