MHB Holly's questions at Yahoo Answers regarding Lagrange multipliers

AI Thread Summary
Holly seeks assistance with Lagrange multipliers for two calculus problems involving optimization with constraints. The first problem involves minimizing the function f(x,y) = x^2 + y^2 under the constraint x + 2y - 20 = 0, leading to a minimum at the point (4,8) with a value of 80. The second problem focuses on maximizing f(x,y) = √(99 - x^2 - y^2) with the constraint x + y - 10 = 0, resulting in a maximum at (5,5) with a value of 7. Evaluations at alternative points confirm these extrema. The discussion effectively illustrates the application of Lagrange multipliers in solving constrained optimization problems.
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Here are the questions:

Calculus 3 Lagrange multipliers help?


Okay I CANNOT figure out lagrange multipliers. Can anyone help me with my homework problems? Thanks!

1.

Use Lagrange multipliers to find the indicated extrema, assuming that x and y are positive.
minimize f(x,y)= x^2 + y ^2
constraint: x+ 2y - 20 = 0

2.
Use Lagrange multipliers to find the indicated extrema, assuming that x and y are positive.

Maximize f(x,y) = (99 - x^2 - y^2)^(1/2)
constraint: x + y - 10 = 0

I have posted a link there to this thread so the OP can view my work.
 
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Hello Holly,

1.) We are given the objective function:

$$f(x,y)=x^2+y^2$$

subject to the constraint:

$$g(x,y)=x+2y-20=0$$

Lagrange multipliers gives rise then to the following system:

$$2x=\lambda(1)$$

$$2y=\lambda(2)$$

This implies:

$$\lambda=2x=y$$

Substituting for $y$ into the constraint, we find:

$$x+2(2x)-20=0\implies x=4\implies y=8$$

Thus, we obtain the critical point:

$$(x,y)=(4,8)$$

The objective function's value at this point is:

$$f(4,8)=4^2+8^2=80$$

To ensure this is a minimum, let's evaluate the objective function at another point on the constraint, such as $$(x,y)=(2,9)$$:

$$f(2,9)=2^2+9^2=85$$

And so we may now conclude:

$$f_{\min}=f(4,8)=80$$

2.) We are given the objective function:

$$f(x,y)=\sqrt{99-x^2-y^2}$$

subject to the constraint:

$$g(x,y)=x+y-10=0$$

Now here we see that $x$ and $y$ have cyclic symmetry, that is, we may switch the two variables and still have the same objective function and constraint. Thus we know the critical value may be obtained when $x=y$. The constraint then gives us:

$$x=y=5$$

And so our critical point is:

$$(x,y)=(5,5)$$

The objective function's value at this point is:

$$f(5,5)=\sqrt{99-5^2-5^2}=7$$

To ensure this is a maximum, let's evaluate the objective function at another point on the constraint, such as $$(x,y)=(4,6)$$:

$$f(5,5)=\sqrt{99-4^2-6^2}=\sqrt{47}<7$$

And so we may conclude:

$$f_{\max}=f(5,5)=7$$
 
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