Respuesta :
Answer:
(a) F(x,y,z,λ) = xyz−λ(480−10xz−10yz−40xy)
Step-by-step explanation:
You want the Lagrangian function for the problem of maximizing the product xyz given the constraint 10xz +10yz +40xy = 480.
Lagrangian
The Lagrangian function for the problem of maximizing or minimizing f(x) subject to the constraint g(x)=0, is written ...
L(x, λ) = f(x) +λ·g(x) . . . . . . where λ is the "Lagrange multiplier"
Application
Here, the function "f(x)" is a function of x, y, z, and the constraint is that the total cost is $480.
f(x, y, z) = xyz . . . . . . the volume of the box, which we want to maximize
g(x, y, z) = 40xy +2(5)(x+y)z -480 . . . . . . . excess of cost over $480
g(x, y, z) = 10xz +10yz +40xy -480
The Lagrangian function for this problem is then ...
F(x, y, z, λ) = xyz -λ(480 -10xz -10yz -40xy) . . . . matches choice A
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Additional comment
The solution to the four equations ∂F/∂x = 0, ∂F/∂y = 0, ∂F/∂z = 0, ∂F/∂λ = 0 is x = y = 2, z = 8, λ = -1/10. The maximum volume is 32 cubic feet.
As with many problems of this sort, you can solve it mentally. The solution that minimizes cost is the one that makes the costs of opposite sides and the bottom each be equal at 1/3 the total. That means the bottom costs $160, so is 160/40 = 4 square feet. The bottom edge of each side is √4 = 2 ft, so the height is $160/($10/ft²)/(2 ft) = 8 ft.