1. Given any set A = {a_1,a_2,a_3,a_4} of four distinct positive integers, we denote the sum a_1 + a_2 + a_3 + a_4 by s_A. Let n_A denote the number of pairs (i,j) with 1 ≦ i ＜ j ≦ 4 for which a_i + a_j divides s_A. Find all sets A of four distinct positive integers which achieve the largest possible value of n_A. 2. Let S be a finite set of at least two points in the plane. Assume that no three points of S are collinear. A windmill is a process that starts with a line L going through a single point P ∈ S. The line rotates clockwise about the pivot P until the first time that the line meets some other point belonging to S. This point, Q, takes over as the new pivot, and the line now rotates clockwise about Q, until it next meets a point of S. This process continues indefinitely. Show that we can choose a point P in S and a line L going through P such that the resulting windmill uses each point of S as a pivot infinitely many times. 3. Let f : R → R be a real-valued function defined on the set of real numbers that satisfies f(x+y) ≦ yf(x) + f(f(x)) for all real numbers x and y. Prove that f(x) = 0 for all x ≦ 0. -- 沒有伴隨著痛苦的教訓是沒意義的。 人如果不犧牲一些東西，就無法得到任何東西。 但是當超越了障礙，並且把得到的東西變成屬於自己的東西時... 人應該就能夠得到無法取代的鋼之心靈吧。 <Fullmetal Alchemist 鋼之鍊金術師> --

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