Question

What is the maximum volume in cubic inches of an open box to be made from a 10- inch by 20-inch piece of cardboard by cutting out squares of equal sides from the four corners and bending up the sides? Your work must include a statement of the function and its derivative.

Answer 1

192.5 in³

Explanation:

The cardboard is 10 by 20 before removing a square from each end. Assuming that the square is x inches wide. Therefore, the 20 in side gets reduced by x inches on both sides, or say it becomes 20 - 2x inches. On the other hand, the 10 in side is also reduced by 2x. The x value we get happens to be the height of the box when the sides are folded up.

Thus the volume V = lbh =

V = (20-2x)*(10-2x)*(x)

V = 4x³ - 60x² + 200x

On differentiating, we have dv/dx to be

dv/dx = 12x² - 120x + 200

Using general formula to find the roots of this equation, we can solve that x = 7.886 and x = 2.113

This roots we got are possible values of x, the square we cut. Since 7.886 * 2 = 15.772 inches, this is more than the 10 inch side, henceforth x = 2.113 inches.

You cut 2.113 inches from each corner to obtain the maximum volume.

The sizes of the cubes are

20 - (2 * 2.113) = 15.774

10 - (2 * 2.113) = 5.774

2.113

The volume of the cube is 15.774 * 5.774 * 2.113 = 192.5 cubic inches.