4. (A1 or B1 problem) Let C be a circle of radius 1, and let D be a diameter of C. Let P be the set of all points inside or on C such that p is closer to D than it is to the circumference of C. Find a rational number r such that the area of P is r. 5. Let n be a positive integer, let 0 ≤ j < n, and let fn(j) be the number of subsets S of the set {0, 1, . . . , n−1} such that the sum of the elem...