the minimum surface area for the box is 4800 cm².
Forming the Equation of SurfaceArea
It is given that the given rectangular box is square-based and top is open. hence, it consists of square and 4 rectangles.
Let the side of the square be a, and height of the box be h.
Then, the total surface area of the box will be given by,
S = a² + 4ah _________ (1)
Also, it is given that the volume of the box is, V = 32000 cm³
The volume of the rectangular box, V = a² h
Eliminating One of the Variables From the Equation
The volume of the rectangular box, V = a² h
⇒ a² h = 32000
⇒ h = 32000/a² _______ (2)
Substituting this value of h in equation (1), we get,
S = a² +4a(32000/a²)
S = a²+128000/a
Minimizing the Surface Area Equation
To find the minima, put, dS/da = 0
dS/da = 2a-128000/a²
⇒ 2a-128000/a² = 0
Multiplying the whole equation with a², we get,
2a³-128000 = 0
⇒ 2a³ = 128000
⇒ a³ = 128000/2
⇒ a³ = 64000
⇒ a = 40 cm
Calculating the Minimum Surface Area
From, equation (2), h = 32000/(40)²
h = 32000/1600
h = 20 cm
Now, substituting the computed values of a and h in equation (1), we get,
S = (40)² +4(40)(20)
S = 1600 +3200
S = 4800 cm²
∴ The minimum surface area of the box is 4800 cm².
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