diameter and height i think
Consider the attached figure. The central square is the "floor" of the box, and it is a (x-2)-by-(x-2) square.
The four rectangles of the side will be the "walls" of the box, and they are (x-2)-by-2 rectangles.
The volume is given by the base area multiplied by the height of the walls, so we have

6 is d because 20/16 simplified is 5/4
7 is d
That is binomial because it has 2 terms
The base case is the claim that

which reduces to

which is true.
Assume that the inequality holds for <em>n</em> = <em>k </em>; that

We want to show if this is true, then the equality also holds for <em>n</em> = <em>k</em> + 1 ; that

By the induction hypothesis,

Now compare this to the upper bound we seek:

because

in turn because
