The 30th International Mathematical Olympiad Shortlist Problems
1989年第30届国际数学奥林匹克备选题 
 Ali Baba has a rectangular piece of carpet. He finds that if he lays if flat on the floor of either of his storerooms then each corner of the carpet touches a different wall of the room. He knows that the storerooms have widths 38 and 50 feet and the same (unknown but integral) length. What are the dimensions of the carpet?
 Prove that for n > 1 the polynomial x^{n}/n! + x^{n1}/(n1)! + ... + x/1! + 1 has no rational roots.
 The polynomial x^{n} + n x^{n1} + a_{2}x^{n2} + ... + a_{0} has n roots whose 16th powers have sum n. Find the roots.
 The angle bisectors of the triangle ABC meet the circumcircle again at A'B'C'. Show that 16 (area A'B'C')^{3} ≥ 27 area ABC R^{4}, where R is the circumradius of ABC.
 Show that any two points P and Q inside a regular ngon can be joined by two circular arcs PQ which lie inside the ngon and meet at an angle at least (1  2/n)π.
 The rectangle R is covered by a finite number of rectangles R_{1}, ... , R_{n} such that (1) each R_{i} is a subset of R, (2) the sides of each R_{i} are parallel to the sides of R, (3) the rectangles R_{i} have disjoint interiors, and (4) each R_{i} has a side of integral length. Show that R has a side of integral length.
 For each n > 0 we write (1 + 2^{1/3}4  4^{1/3}4)^{n} as a_{n} + b_{n} 2^{1/3} + c_{n} 4^{1/3}, where a_{n}, b_{n}, c_{n} are integers. Show that c_{n} is nonzero.
 Let C represent the complex numbers. Let g: C → C be an arbitrary function. Let w be a cube root of 1 other than 1 and let v be any complex number. Find a function f: C → C such that f(z) + f(wz + v) = g(z) for all z and show that it is unique.
 Define the sequence a_{1}, a_{2}, a_{3}, ... by 2^{n} = the sum of a_{d} such that d divides n. Show that a_{n} is divisible by n. [For example, a_{1} = 2, a_{2} = 2, a_{3} = 6.]
 There are n cars waiting at distinct points of a circular race track. At the starting signal each car starts. Each car may choose arbitrarily which of the two possible directions to go. Each car has the same constant speed. Whenever two cars meet they both change direction (but not speed). Show that at some time each car is back at its starting point.
 A quadrilateral has both a circumcircle and an incircle. Show that intersection point of the diagonals lies on the line joining the centers of the two circles.
 a, b, c, d, m, n are positive integers such that a^{2} + b^{2} + c^{2} + d^{2} = 1989, a + b + c + d = m^{2} and the largest of a, b, c, d is n^{2}. Find m and n.
 The real numbers a_{0}, a_{1}, ... , a_{n} satisfy a_{0} = a_{n} = 0, a_{k} = c + ∑_{i=k}^{n1} a_{ik}(a_{i} + a_{i+1}). Show that c ≤ 1/(4n).
 Given 7 points in the plane, how many segments (each joining two points) are needed so that given any three points at least two have a segment joining them?
 Given a convex ngon A_{1}A_{2} ... A_{n} with area A and a point P, we rotate P through an angle x about A_{i} to get the point P_{i}. Find the area of the polygon P_{1}P_{2} ... P_{n}.
 A positive integer is written in each square of an m x n board. The allowed move is to add an integer k to each of two adjacent numbers (whose squares have a common side). Find a necessary and sufficient condition for it to be possible to get all numbers zero by a finite sequence of moves.
 Show that the intersection of a plane and a regular tetrahedron can be an obtuseangled triangle, but that the obtuse angle cannot exceed 120^{o}.
 Five points are placed on a sphere of radius 1. That is the largest possible value for the shortest distance between two of the points? Find all configurations for which the maximum is attained.
 a and b are nonsquare integers. Show that x^{2}  ay^{2}  bz^{2} + abw^{2} = 0 has a solution in integers not all zero iff x^{2}  ay^{2}  bz^{2} = 0 has a solution in integers not all zero.
 b > 0 and a are fixed real numbers and n is a fixed positive integer. The real numbers x_{0}, x_{1}, ... , x_{n} satisfy x_{0} + x_{1} + ... + x_{n} = a and x_{0}^{2} + ... + x_{n}^{2} = b. Find the range of x_{0}.
 Let m > 1 be a positive odd integer. Find the smallest positive integer n such that 2^{1989} divides m^{n}  1.
 The points O, A, B, C, D in the plane are such that the six triangles OAB, OAC, OAD, OBC, OBD, OCD all have area at least 1. Show that one of the triangles must have area at least √2.
 155 birds sit on a circle center O. Birds at A and B are mutually visible iff ∠AOB ≤ 10^{o}. More than one bird may sit at the same point. What is the smallest possible number of mutually visible pairs?
 Given positive integers a ≥ b ≥ c, let N(a, b, c) be the number of solutions in positive integers x, y, z to a/x + b/y + c/z = 1. Show that N(a, b, c) ≤ 6ab(3 + ln(2a) ).
 ABC is an acuteangled triangle with circumcenter O and orthocenter H. AO = AH. Find all possible values for the ∠A.

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