Question

According to u.s. postal regulations, the girth plus the length of a parcel sent by mail may not exceed 108 inches, where by "girth" we mean the perimeter of the smallest end. what is the largest possible volume of a rectangular parcel with a square end that can be sent by mail? such a package is shown below. assume y>x. what are the dimensions of the package of largest volume?

Answer 1

Let x represent the side length of the square end, and let d represent the dimension that is the sum of length and girth. Then the volume V is given by
  V = x²(d -4x)
Volume will be maximized when the derivative of V is zero.
  dV/dx = 0 = -12x² +2dx
  0 = -2x(6x -d)
This has solutions
  x = 0, x = d/6

a) The largest possible volume is
  (d/6)²(d -4d/6) = 2(d/6)³
  = 2(108 in/6)³ = 11,664 in³

b) The dimensions of the package with largest volume are
  d/6 = 18 inches square by
  d -4d/6 = d/3 = 36 inches long