Answer:
20 by 20 by 20
Step-by-step explanation:
Let the total surface of the rectangular box be expressed as S = 2xy + 2yz + 2xz
x is the length of the box
y is the width and
z is the height of the box.
S = 2xy + 2yz + 2xz ... 1
Given the volume V = xyz = 8000 ... 2
From equation 2;
z = 8000/xy
Substituting into equation 1;
S = 2xy + 2y(8000/xy)+ 2x(8000/xy)
S = 2xy+16000/x+16000/y
Differentiating the resulting equation with respect to x and y will give;
dS/dx = 2y + (-16000x⁻²)
dS/dx = 2y - 16000/x²
Similarly,
dS/dy = 2x + (-160000y⁻²)
dS/dy = 2x - 16000/y²
Note that at the turning point, ds/dx = 0 and ds/dy = 0, hence;
2y - 16000/x² = 0 and 2x - 16000/y² = 0
2y = 16000/x² and 2x = 16000/y²
2y = 16000/(8000/y²)²
2y = 16000×y⁴/64,000,000
2y = y⁴/4000
y³ = 8000
y =³√8000
y = 20
Given 2x = 16000/y²
2x = 16000/20²
2x = 16000/400
2x = 40
x = 20
Since Volume of the box is V = xyz
8000 = 20(20)z
8000 = 400z
z = 8000/400
z = 20
Hence, the dimensions which minimize the surface area of this box is 20 by 20 by 20.
