Answer:
Dimensions of the box:
x = 1,26 ( the side of the square base )
h = 2,52 the heigh of the cube
Step-by-step explanation:
V(b) = x² *h x is the side of the square base, and h the heigh of the cube
The surface area of such cube is:
Area of the base A(b) = x²
Lateral area is 4*x*h
A(c) = x² + 4*x*h
Now x² * h = v = 4 m³ ⇒ h = 4/x² and
A(x) = x² + 4/x
Tacking derivatives on both side of the equation:
A´(x) = 2*x - 4/x²
A´(x) = 0 2*x - 4/x² = 0 ⇒ 2*x³ - 4 = 0
x³ = 2
x = 1,26 m
And h = 4/(1,26)² ⇒ h = 2,52 m
How do we know the value of x = 1,26 is for a minimum value of A(x)
We find the second derivative of A(x)
A´´(x) = 2 - (-4*2*x/x⁴)
A´´(x) = 2 + 8/x³ is positive A´´(x) > 0
Then function A(x) has a minimum value at x = 1,26
