The 32nd International Mathematical Olympiad Shortlist Problems
1991年第32届国际数学奥林匹克赛备选题 
 ABC is a triangle and P an interior point. Let the feet of the perpendiculars from P to AC, BC be P_{1}, P_{2} respectively, and let the feet of the perpendiculars from C to AP, BP be Q_{1}, Q_{2} respectively. Show that P_{1}Q_{2} and P_{2}Q_{1} meet on the line AB.
 ABC is an acuteangled triangle. M is the midpoint of BC and P is the point on AM such that MB = MP. H is the foot of the perpendicular from P to BC. The lines through H perpendicular to PB, PC meet AB, AC respectively at Q, R. Show that BC is tangent to the circle through Q, H, R at H.
 If S is a point on the circumcircle of the triangle PQR, show that the feet of the perpendiculars from S to the lines PQ, QR, RP are collinear. Denote the line by [S, PQR]. If ABCDEF is a hexagon whose vertices lie on a circle, show that the lines [A, BDF], [B, ACE], [D, ABF], [E, ABC] are concurrent iff CDEF is a rectangle.
 was used in the Olympiad.
 ABC is a triangle with ∠A = 60^{o} and incenter I. P lies on BC with 3 BP = BC. The point F lies on AB and IF is parallel to AC. Show that ∠BFP = ∠FBI.
 was used in the Olympiad.
 O is the circumcenter of the tetrahedron ABCD. The midpoints of BC, CA, AB are L, M, N respectively. AB + BC = AD + CD, CB + CA = BD + AD and CA + AB = CD + BD. Show that ∠LOM = ∠MON = ∠NOL.
 Given a set A of n points in the plane, no three collinear, show that we can find a set B of 2n  5 points such that a point of B lies in the interior of every triangle whose vertices belong to A.
 A graph has 1991 points and every point has degree at least 1593. Show that there are six points, each of which is joined to the others. Is 1593 the best possible?
 was used in the Olympiad.
 Let mCn represent the binomial coefficient m!/(n! (mn)! ). Prove that (1991C0)/1991  (1990C1)/1990 + (1989C2)/1989  (1988C3)/1988 + ...  (996C995)/996 = 1/1991.
 was used in the Olympiad.
 No term of the sequence a_{1}, a_{2}, ... , a_{n} is divisible by n and the sum of all n terms is not divisible by n. Show that for n > 1 there are at least n different subsequences whose sum is divisible by n.
 The quadratic ax^{2} + bx + c has integer coefficients. For some prime p its values are squares for 2p1 consecutive integers x. Show that p divides b^{2}  4ac.
 Let a_{n} be the last nonzero digit of n! Does the sequence a_{n} become periodic after a finite number of terms?
 was used in the Olympiad.
 Find all positive integer solutions to 3^{m} + 4^{n} = 5^{k}.
 What is the largest power of 1991 which divides 1990^{m} + 1992^{n}, where m = 1991^{1992} and n = 1991^{1990} ?
 Show that the only rational solution q in the range 0 < q < 1 to the equation cos(3πq) + 2 cos(2πq) = 0 is 2/3.
 Let k be the positive root of the equation x^{2} = 1991 x + 1. For positive integers m, n define m # n = mn + [km] [kn]. Show that the operation # is associative.
 The polynomial p(x) = x^{1991} + a_{1990}x^{1990} + ... + a_{0} has integer coefficients. Show that the equation p(x)^{2} = 9 has at most 1995 distinct integer solutions.
 There is exactly one square with its vertices on the curve y = x^{3} + ax^{2} + bx + c, where a, b, c are real. Show that the square must have side 72^{1/4}.
 f(n) is an integervalued function defined on the integers which satisfies f(m + f( f(n) ) ) =  f( f(m+1))  n for all m, n. The polynomial g(n) has integer coefficients and g(n) = g( f(n) ) for all n. Find f(1991) and the most general form for g.
 Find all odd integers n > 1 for which there is at least one permutation a_{1}, a_{2}, ... , a_{n} of 1, 2, 3, ... , n such that the sums
a_{1}  a_{2} + a_{3}  ... + a_{n},
a_{2}  a_{3} + a_{4}  ...  a_{n} + a_{1},
a_{3}  a_{4} + ... + a_{n}  a_{1} + a_{2},
...
a_{n}  a_{1} + a_{2}  ... + a_{n1}
are all positive.
 Given n > 1 real numbers 0 ≤ x_{i} ≤ 1, show that for some i < n we have x_{i}(1  x_{i+1}) ≥ x_{1}(1  x_{n})/4.
 a_{1}, a_{2}, ... , a_{n} and b_{1}, b_{2}, ... , b_{n} are nonnegative reals such that a_{1} + a_{2} + ... + a_{n} = b_{1} + b_{2} + ... + b_{n} = 1. Let p_{i} be the product a_{1}a_{2} ... a_{i1}b_{i}a_{i+1} ... a_{n}. If none of the b_{i} exceed k, where 1/2 ≤ k ≤ 1, show that p_{1} + p_{2} + ... + p_{n} ≤ k/(n  1)^{n1} .
 Find the maximum possible value of x_{1}x_{2}(x_{1} + x_{2}) + x_{1}x_{3}(x_{1} + x_{3}) x_{1}x_{2}(x_{1} + x_{2}) + x_{1}x_{4}(x_{1} + x_{4}) + ... + x_{1}x_{n}(x_{1} + x_{n}) for nonnegative real numbers x_{1}, x_{2}, ... , x_{n} with sum 1.
 was used in the Olympiad.
 Find all subsets X of the real line such that for any stretch S there is a translation T such that S(X) = T(X). A stretch is a transformation x → h + k(x  h), where h and k ≠ 0 are constants, and a translation is a transformation x → x + t, where t is a constant.
 Two people A and B and an umpire U play the following game. A chooses an arbitrary positive integer m, and B chooses an arbitrary positive integer n. Each gives his integer to U but not to the other player. U then writes two integers on a blackboard, one of which is m + n. U asks A if he knows n. If he does not, then U asks B if he knows m. If he does not, then U asks A if he knows n, and so on. Show that one player will be able to deduce the other's number after a finite number of questions.

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