Question

A box is to be constructed from a sheet of cardboard that is 20 cm by 50 cm by cutting out squares of length x by x from each corner and bending up the sides. What is the maximum volume this box could have? (Round your answer to two decimal places. Do not include units, for example, 10.22 cm would be 10.22.

Answer 1

We are given the dimensions of the box:

l = 50 cm

w = 20 cm

 

We know that the volume of a box is:

V = l w h

where h = x

 

However since x cuts is made on both all sides of the box, therefore the new dimensions would be:

V = (l – 2x) (w – 2x) x

V = (50 – 2x) (20 – 2x) x

V = 1000x – 100x^2 – 40x^2 + 4x^3

V = 4x^3 – 140x^2 + 1000x

 

To get the maxima value, we get the 1st derivative of the function then set dV/dx = 0 to solve for x:

dV / dx = 12x^2 – 280x + 1000

12x^2 – 280x + 1000 = 0

Transpose 1000 to the right side and divide everything by 12:

x^2 – (280/12)x = -(1000/12)

Completing the square:

x^2 – (280/12)x + (78400/576) = -(1000/12) + (78400/576)

[x – (280/24)]^2 = 52.78

x – (280/24) = ±7.26

x =  (280/24) ± 7.26

x = 4.40, 18.93

x cannot be 18.93 since this would result in a negative value of 20 – 2x, therefore:

x = 4.40 cm

 

Calculating for the volume:

V = (50 – 2*4.4) (20 – 2*4.4) (4.4)

V = 2030.34 cm^3