K. 392. An underground supporting cable is to be fixed to a pole that will be entirely under ground, even the top of the pole will be buried. When the pit dug for the pole was half done, the workers lowered the pole into the pit. They made the following observation: When the whole pit is dug out, the top of the pole will be exactly twice as much below ground level as it is sticking out of the pit now. Given that the height of the pole is two metres, find the depth of the pit when it is completely dug out.
K. 393. Consider a product whose factors are all 7. Is it possible that the result is a 45-digit number in which the digits 1, 2, 3, 4, 5, 6, 7, 8, 9 occur 9, 8, 7, 6, 5, 4, 3, 2, 1 times, respectively?
C. 1191. A truckload of goods consists of n packages weighing 1 kg, n-1 packages weighing 2 kg, n-2 packages of 3 kg, and so on, finally there is 1 package of n kg. Express the average mass of a package as a function of n.
C. 1193. An isosceles triangle of apex angle 120o is drawn on each side of a unit square, on the outside of the square. An equilateral octagon is obtained. In comparison, consider the regular octagon in which the side of the square formed by connecting every other vertex is unity. Find the ratio of the area of the equilateral octagon to the area of the regular octagon.
C. 1195. Let a, b and c, respectively, be the lengths of the face diagonals AF, FC and CA of a cuboid ABCDEFGH. Let , and , respectively, denote the angles enclosed by the face diagonals with the diagonal BH of the cuboid. Prove that a.cos -b.cos +c.cos =0 if the length of edge AB is between the lengths of the edges BC and BF.
B. 4575. According to the tradition of the ancient tribe of Head Counters, every year is designated either as lucky or as sinister. For example, 2013 is a lucky year since the first 2013 positive integers can be partitioned into at least two classes such that each class contains the same number of elements and the sum of the elements is also equal in each class. If such a partition does not exist, the year is said to be sinister. Which years are sinister?
B. 4576. Six distinct points lie on a circle. Three points are selected, and the orthocentre of their triangle is connected to the centroid of the triangle formed by the other three points. Prove that all line segments obtained in this way are concurrent.
B. 4579. Represent those pairs (a;b) of real numbers in the rectangular coordinate plane for which the polynomial x(x+4)+a(y2-1)+2by of two variables can be expressed as a product of two first-degree polynomials.
A. 601. Let q1 be an integer. Prove that there is an integer Cq such that for every finite set A of integers |A+q.A|(q+1)|A|-Cq holds. (A+q.A is the set of those integers that can be expressed as a+qa' with some a,a'A.)