
Problems with sign 'C'
Deadline expired on 10 June 2011. 
C. 1080. A steerable airship has two engines and a given supply of fuel. If both engines are operated, the airship covers 88 kilometres per hour. If the first engine were used only, the fuel would last 25 hours longer, but the distance covered per hour would only be 45 km. If the second engine were used only, the fuel would last 16 hours longer than with two engines, and the distance covered per hour would only be 72 km. In which case can the airship travel the longest distance?
(5 pont)
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C. 1081. Two regular polygons are said to be matching if the double of the interior angle of one of them equals the triple of the exterior angle of the other. Find all pairs of matching polygons.
(5 pont)
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C. 1082. The first digit of a sixdigit number is transferred to the end of the number. Then the first digit of the resulting sixdigit number is, again, transferred to the end of the number. The sixdigit number obtained in this way is three times the previous number and times the original number. What is the original sixdigit number?
(5 pont)
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C. 1083. The length of one side of a triangle is 8 cm, one of the angles lying on it is 60^{o}, and the radius of the inscribed circle is cm. How long are the other two sides?
(5 pont)
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C. 1084. The point P(8;4) divides a chord of the parabola y^{2}=4x in a 1:4 ratio. Find the coordinates of the endpoints of the chord.
(5 pont)
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Problems with sign 'B'
Deadline expired on 10 June 2011. 
B. 4362. Each vertex of a solid cube is cut off. The faces of the resulting polyhedron are 8 triangles and 6 heptagons. What are the possible numbers of vertices and edges of such polyhedra?
(3 pont)
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B. 4363. The reciprocals of the natural numbers 2 to 2011 are written on a blackboard. In each step, two numbers x and y are erased and replaced with the number . This step is repeated 2009 times, until a single number remains. What may the remaining number be?
(Suggested by B. Kovács, Szatmárnémeti)
(4 pont)
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B. 4364. Let abc>0. Prove that .
(Suggested by J. Mészáros, Jóka)
(4 pont)
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B. 4365. Find all positive integers n such that 2^{n}1 and 2^{n+2}1 are both primes, and 2^{n+1}1 is not divisible by 7.
(Suggested by S. Kiss, Budapest)
(3 pont)
solution (in Hungarian), statistics
B. 4366. Let M denote the orthocentre of the acuteangled triangle ABC, and let A_{1}, B_{1}, C_{1}, respectively, denote the circumcentres of triangles BCM, CAM, ABM. Prove that the lines AA_{1}, BB_{1} és CC_{1} are concurrent.
(4 pont)
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B. 4367. Solve the following equation: .
(Suggested by J. Mészáros, Jóka)
(4 pont)
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B. 4368. Let D, E and F, respectively, be points on sides AB, BC, CA of a triangle ABC such that AD:DB=BE:EC=CF:FA1. The lines AE, BF, CD intersect one another at points G, H, I, respectively. Prove that the centroids of triangles ABC and GHI coincide.
(Suggested by Sz. Miklós, Herceghalom)
(3 pont)
solution (in Hungarian), statistics
B. 4369. Each of the circles k_{1}, k_{2} and k_{3} passes through a point P, and the circles k_{i} and k_{j} also pass through the point M_{i,j}. Let A be an arbitrary point of circle k_{1}. Let k_{4} be an arbitrary circle passing through A and M_{1,2}, and let k_{5} be an arbitrary circle passing through A and M_{1,3}. Show that if the other intersections of the pairs of circles k_{4} and k_{2}, k_{5} and k_{3}, k_{4} and k_{5} are B, C and D, respectively, then the points M_{2,3}, B, C, D are either concyclic or collinear.
(4 pont)
solution (in Hungarian), statistics
B. 4370. Let a, b, c denote the lengths of the sides of a triangle, and let u, v, w, respectively, be the distances of the centre of the incircle from the vertices opposite to the sides. Prove that .
(Suggested by J. Mészáros, Jóka)
(5 pont)
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B. 4371. Prove that
(Suggested by B. Kovács, Szatmárnémeti)
(5 pont)
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Problems with sign 'A'
Deadline expired on 10 June 2011. 
A. 536. The positive real numbers a, b, c, d satisfy a+b+c+d=abc+abd+acd+bcd. Prove that
(5 pont)
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A. 537. The edges of the complete graph on n vertices are labeled by the numbers in such a way that each number is used exactly once. Prove that if n is sufficiently large then there exists a (possible cyclic) path of three edges such that the sum of the numbers assigned to these edges is at most 3n1000.
(Kolmogorov Cup, 2009; a problem by I. Bogdanov, G. Chelnokov and K. Knop)
(5 pont)
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A. 538. In the 3dimensional hyperbolic space there are given a plane and four distinct lines a_{1}, a_{2}, r_{1}, r_{2} in such positions that a_{1} and a_{2} are perpendicular to , r_{1} is coplanar with a_{1}, r_{2} is coplanar with a_{2}, finally r_{1} and r_{2} intersect at the same angle. Rotate r_{1} around a_{1} and rotate r_{2} around a_{2}; denote by and the two surfaces of revolution they sweep out. Show that the common points of and lie in a plane.
(5 pont)
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