Answer:
The dimensions of the box are;
2 feet, by 2 feet, by 4 feet
Explanation:
The given parameters are;
The volume of the closed box = 16 ft³
The material with which the top is made = The material with which the bottom is made
The cost of the material with which the top and bottom is made = 10 cents/(ft.²)
The cost of the material with which the side is made = 5 cents/(ft.²)
Let 'x', 'y', and 'z' represent the length, width, and height of the material, we have;
x·y·z = 16...(1)
The total cost of the box = 2×10×x×y + 2×5×x×z + 2×5×y×z
∴ The total cost of the box = 20·x·y + 10·x·z + 10y·z...(2)
From equation (1)
z = 16/(x·y)
Therefore, we have;
20·x·y + 10·x·z + 10y·z. = 20·x·y + 10·x·16/(x·y) + 10y·16/(x·y)
20·x·y + 160/(y) + 160/(x)
Differentiating, we get;
f'(x) = 20·y - 160/x²
f'(y) = 20·x - 160/y²
20·y - 160/x² = 0
y = 8/x²
20·x - 160/y² = 0
x = 8/y²
∴ x = 8/(8/x²)² = x⁴/8
8 = x⁴/x = x³
x = ∛8 = 2
The length, x = 2 feet
y = 8/x² = 8/2² = 2
The width, y = 2 feet
z = 16/(x·y) = 16/(2 × 2) = 4
The height of the box, z = 4 feet
Therefore, the dimensions of the box are 2 feet, by 2 feet, by 4 feet.