Suppose you are asked to design a rotating disk where the number of bits per track is constant. You know that the number of bits per track is determined by the circumference of the innermost track, which you can assume is also the circumference of the hole. Thus, if you make the hole in the center of the disk larger, the number of bits per track increases, but the total number of tracks decreases. If you let r denote the radius of the platter, and x * r the radius of the hole, what value of x maximizes the capacity of the disk?