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The 8th All Soviet Union Mathematical Olympiad
1974年第八届全苏数学奥林匹克
  1. A collection of n cards is numbered from 1 to n. Each card has either 1 or -1 on the back. You are allowed to ask for the product of the numbers on the back of any three cards. What is the smallest number of questions which will allow you to determine the numbers on the backs of all the cards if n is (1) 30, (2) 31, (3) 32? If 50 cards are arranged in a circle and you are only allowed to ask for the product of the numbers on the backs of three adjacent cards, how many questions are needed to determine the product of the numbers on the backs of all 50 cards ?
  2. Find the smallest positive integer which can be represented as 36m - 5n.
  3. Each side of a convex hexagon is longer than 1. Is there always a diagonal longer than 2? If each of the main diagonals of a hexagon is longer than 2, is there always a side longer than 1 ?
  4. Circles radius r and R touch externally. AD is parallel to BC. AB and CD touch both circles. AD touches the circle radius r, but not the circle radius R, and BC touches the circle radius R, but not the circle radius r. What is the smallest possible length for AB ?
  5. Given n unit vectors in the plane whose sum has length less than one. Show that you can arrange them so that the sum of the first k has length less than 2 for every 1 < k < n.
  6. Find all real a, b, c such that |ax + by + cz| + |bx + cy + az| + |cx + ay + bz| = |x + y + z| for all real x, y, z.
  7. ABCD is a square. P is on the segment AB and Q is on the segment BC such that BP = BQ. H lies on PC such that BHC is a right angle. Show that DHQ is a right angle.
  8. The n points of a graph are each colored red or blue. At each move we select a point which differs in color from more than half of the points to which it it is joined and we change its color. Prove that this process must finish after a finite number of moves.
  9. Find all positive integers m, n such that nn has m decimal digits and mm has n decimal digits.
  10. In the triangle ABC, angle C is 90 deg and AC = BC. Take points D on CA and E on CB such that CD = CE. Let the perpendiculars from D and C to AE meet AB at K and L respectively. Show that KL = LB.
  11. One rat and two cats are placed on a chess-board. The rat is placed first and then the two cats choose positions on the border squares. The rat moves first. Then the cats and the rat move alternately. The rat can move one square to an adjacent square (but not diagonally). If it is on a border square, then it can also move off the board. On a cat move, both cats move one square. Each must move to an adjacent square, and not diagonally. The cats win if one of them moves onto the same square as the rat. The rat wins if it moves off the board. Who wins? Suppose there are three cats (and all three cats move when it is the cats' turn), but that the rat gets an extra initial turn. Prove that the rat wins.
  12. Arrange the numbers 1, 2, ... , 32 in a sequence such that the arithmetic mean of two numbers does not lie between them. (For example, ... 3, 4, 5, 2, 1, ... is invalid, because 2 lies between 1 and 3.) Can you arrange the numbers 1, 2, ... , 100 in the same way ?
  13. Find all three digit decimal numbers a1a2a3 which equal the mean of the six numbers a1a2a3, a1a3a2, a2a1a3, a2a3a1, a3a1a2, a3a2a1.
  14. No triangle of area 1 can be fitted inside a convex polygon. Show that the polygon can be fitted inside a triangle of area 4.
  15. f is a function on the closed interval [0, 1] with non-negative real values. f(1) = 1 and f(x + y) ≥ f(x) + f(y) for all x, y. Show that f(x) ≤ 2x for all x. Is it necessarily true that f(x) ≤ 1.9x for all x.
  16. The triangle ABC has area 1. D, E, F are the midpoints of the sides BC, CA, AB. P lies in the segment BF, Q lies in the segment CD, R lies in the segment AE. What is the smallest possible area for the intersection of triangles DEF and PQR ?
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