New exercises and problems in Mathematics September
2003
 New exercises Maximum
score for each exercise (sign "C") is 5 points. 
C. 725. A coin has been placed in each field of a 3x3 table, showing tails on top. At least how many coins need to be turned over, so that there are no three collinear (row, column, diagonal) heads or three collinear tails?
C. 726. Is there a regular polygon in which the shortest diagonal equals the radius of the circumscribed circle?
C. 727. Peter's telephone number (without area code) is 312837, that of Paul is 310650. If each of these numbers is divided by the same threedigit number, the remainders will be equal. That remainder is the area code of their city. What is the remainder? (Note: Area codes are twodigit numbers in Hungary.)
C. 728. The angles A and B of a convex quadrilateral ABCD are equal, and angle C is a right angle. The side AD is perpendicular to the diagonal BD. The lengths of sides BC and CD are equal. What is the ratio between their common length and the length of side AD?
C. 729. Solve the equation 2x log x +x 1 = 0 on the set of real numbers. (Suggested by É. Gyanó, Budapest)
 New problemsThe maximum
scores for problems (sign "B") depend on the difficulty. It is allowed
to send solutions for any number of problems, but your score will be
computed from the 6 largest score in each month. 
B. 3652. We have coloured each positive integer either red or blue. The sum of two numbers of different colours is always blue, and their product is always red. What colour is the product of two red numbers? (3 points)
B. 3653. Find the locus of those points in the plane of a given square at which the square subtends an angle of 30^{o}. (3 points)
B. 3654. Prove that if m and n are integers, m^{2}+n^{2}+m+n1 cannot be divisible by 9. (3 points)
B. 3655. The convex hexagon ABCDEF is cyclic, and AB=BC=a, CD=DE=b, EF=FA=c. Prove that the area of the triangle BDF is half of the area of the hexagon. (4 points)
B. 3656. The number F in basea notation is \(\displaystyle 0{,}3737\ldots=
0{,}\dot3\dot7\) (the dots denoting the beginning and the end of the recurring sequence of digits), and the number G in basea notation is \(\displaystyle 0{,}7373\ldots=0{,}\dot7\dot3\). The same numbers written in baseb notation are \(\displaystyle F=0{,}2525\ldots=
0{,}\dot2\dot5\) and \(\displaystyle G=0{,}5252\ldots=0{,}\dot5\dot2\). Determine the numbers a and b. (4 points)
B. 3657. Is there a rightangled triangle such that the radius of the incircle and the radii of the three excircles are four consecutive terms of an arithmetic progression? (4 points)
B. 3658. The point P lies on the perpendicular line segment dropped from the vertex A of the regular tetrahedron ABCD onto the face BCD. The lines PB, PC and PD are pairwise perpendicular. In what ratio does P divide the perpendicular line segment? (3 points)
B. 3659. Given the real number t, write the expression x^{4}+tx^{2}+1 as a product of two quadratic factors of real coefficients. (4 points)
B. 3660. The points X, Y and Z divide a circle into three arcs that subtend angles of 60^{o}, 100^{o} and 200^{o} at the centre of the circle. If A, B and C are the vertices of a triangle, let M_{A} and M_{B} denote the intersections of the altitudes drawn from the vertices A and B with the circumscribed circle, and let F_{C} denote the intersection of the bisector of angle C with the circumscribed circle. Determine all the acute triangles ABC for which the points M_{A}, M_{B} and F_{C} coincide with the points X, Y and Z in some order. (4 points)
B. 3661. Let x_{1}=1, y_{1}=2, z_{1}=3, and let \(\displaystyle x_{n+1}=y_n+
\frac{1}{z_n}\), \(\displaystyle y_{n+1}=z_n+\frac{1}{x_n}\), \(\displaystyle z_{n+1}=x_n+
\frac{1}{y_n}\) for every positive integer n. Prove that at least one of the numbers x_{200}, y_{200} and z_{200} is greater than 20. (5 points)
 New advanced problems Maximum score
for each advanced problem (sign "A") is 5 points. 
A. 323. I is the isogonic point of a triangle ABC (the point in the interior of the triangle for which \(\displaystyle \angle\)AIB=\(\displaystyle \angle\)BIC=\(\displaystyle \angle\)CIA=120^{o}). Prove that the Euler lines of the triangles ABI, BCI and CAI are concurrent.
A. 324. Prove that if a,b,c are positive real numbers then
\(\displaystyle
\frac{1}{a(1+b)}+\frac{1}{b(1+c)}+\frac{1}{c(1+a)}\ge\frac{3}{1+abc}.
\)
A. 325. We have selected a few 4element subsets of an nelement set A, such that any two sets of four elements selected have at most two elements in common. Prove that there exists a subset of A that has at least \(\displaystyle \root3\of{6n}\) elements and does not contain any of the selected 4tuples as a subset.
Send your solutions to the following address:
KöMaL Szerkesztőség (KöMaL feladatok), Budapest 112,
Pf. 32. 1518, Hungary or by email to:
Deadline: 15 October 2003
