Junior problems. = n 2 n N, 2

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1 Junior problems J35. For positive real numbers a and b, define their perfect mean to be half of the sum of their arithmetic and geometric means. Find how many unordered pairs of integers (a, b) from the set {1,,..., 015} have their perfect mean a perfect square. Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by Alessandro Ventullo, Milan, Italy i.e. We have a+b + ab = n n N, a + b = n, n N. As a, b {1,,..., 015}, then a and b must be perfect squares and n 44. Assume that a b. If a is an odd perfect square, then b is an odd perfect square, and making a case by case analysis for a {1, 3,..., 43 } we get = 53 unordered pairs. Similarly, if a is an even perfect square, then b is an even perfect square and we get = 53 unordered pairs. So, there are = 506 unordered pairs which satisfy the given conditions. Also solved by Yujin Kim, The Stony Brook School, NY, USA; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Arber Igrishta, Eqrem Qabej, Vushtrri, Kosovo; Behzod Ibodullaev, Lyceum TCTU, Tashkent, Uzbekistan; David E. Manes, Oneonta, NY, USA; Dilshoda Ibragimova, Academic Lyceum Nr. under the SamIES, Samarkand, Uzbekistan; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Francesc Gispert Sanchez, CFIS, Universitat Politecnica de Catalunya, Barcelona, Spain; Haroun Meghaichi, University of Science and Technology Houari Boumediene, Algiers, Algeria; Joseph Lee, Loomis Chaffee School in Windsor, CT, USA; Luiz Ernesto, M.A.Leitão, Brasil; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Titu Zvonaru and Neculai Stanciu, Romania; Ji Eun Kim, Tabor Academy, MA, USA; Misiakos Panagiotis, Athens College(HAEF), Greece; Michael Tang, Edina High School, MN, USA; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; Myong Claire Yun, Seoul International School, South Korea. Mathematical Reflections 1 (015) 1

2 J36. Let a, b, c be nonnegative integers. Prove that a + 3b + 4c + 3a + 4b + c + 4a + b + 3c ( a + b + c). Proposed by by Titu Andreescu, University of Texas at Dallas, USA Solution by Brian Bradie, Christopher Newport University, Newport News, VA, USA Note that a a + 3b + 4c = b 9 + 4c 9 1 (a + 3b + 4c) 3 by Jensen s inequality. Similarly, 3a + 4b + c 1 (3a + 4b + c) 3 and 4a + b + 3c 1 (4a + b + 3c). 3 Therefore, a + 3b + 4c + 3a + 4b + c + 4a + b + 3c 3(a + b + c) = 9 ( a) + ( b) + ( c) 3 ( a ) + b + c 9 3 = ( a + b + c), where the final inequality follows from the quadratic mean-arithmetic mean inequality. Also solved by George Gavrilopoulos, High School Of Nea Makri, Athens, Greece; Yong Xi Wang,East China Institute Of Technology, China; AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Utsab Sarkar, Chennai Mathematical Institute, India; Titu Zvonaru and Neculai Stanciu, Romania; Stefan Petrevski, NOVA International School in Skopje, Macedonia; Albert Stadler, Herrliberg, Switzerland; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Rade Krenkov, SOU "Goce Delcev", Valandovo, Macedonia; Ángel Plaza, University of Las Palmas de Gran Canaria, Spain; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Odilbek Utamuratov, Uzbekistan; Nicuşor Zlota, Traian Vuia Technical College, Focşani, Romania; Haroun Meghaichi, University of Science and Technology Houari Boumediene, Algiers, Algeria; Francesc Gispert Sanchez, CFIS, Universitat Politecnica de Catalunya, Barcelona, Spain; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Dilshoda Ibragimova, Academic Lyceum Nr. under the SamIES, Samarkand, Uzbekistan; Behzod Ibodullaev, Lyceum TCTU, Tashkent, Uzbekistan; David E. Manes, Oneonta, NY, USA; Arkady Alt, San Jose, CA, USA; Arber Igrishta, Eqrem Qabej, Vushtrri, Kosovo; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Myong Claire Yun, Seoul International School, South Korea; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; Michael Tang, Edina High School, MN, USA; Misiakos Panagiotis, Athens College(HAEF), Greece; Ji Eun Kim, Tabor Academy, MA, USA; Yujin Kim, The Stony Brook School, NY, USA. Mathematical Reflections 1 (015)

3 J37. A jeweler makes a circular necklace out of nine distinguishable gems: three sapphires, three rubies and three emeralds. No two gems of the same type can be adjacent to each other and necklaces obtained by rotation and reflection (flip) are considered to be identical. How many different necklaces can she make? Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by Myong Claire Yun, Seoul International School, South Korea Start by placing the three sapphires S 1, S, S 3 as shown in the figure below. There will be three gaps in between to fill with emeralds and rubies. The distribution of the mix jewels in the 3 gaps can be categorized as: Type 1 : ( ) or ( ) or ( ) Type : ( ) and the permutations Type 3 : ( + + ) Let s count separately the number of necklaces of each type: Type 1 : ( ): We need rubies and emeralds to fill in gap 1. ( )( ) 3 3 = 9 ways to select these 4 jewels. They can be arranged in = 8 ways. The remaining two gaps can be filled out in ways. Hence these are 9 8 = 144 necklaces of type ( ). Type : ( ): We need rubies and 1 emerald or 1 ruby and emeralds to fill in gap 1. ( )( ) 3 3 = 18 1 ways. Arranging can be done in ways. Gap 3 can be gilled out in ways. We are left with one ruby abd one emerald to place in gap ways. Therefore there is 18 = 144 necklaces of type ( ). Mathematical Reflections 1 (015) 3

4 Type 3 : ( + + ): ( )( ) 3 3 = ways to fill gap 1. Once the first gap is taken care of, there is ( )( ) 3 = ways to fill out gap. Therefore, there are 18 8 = 88 necklaces of type ( + + ). In conclusion, there is = 1584 different necklaces. Also solved by Yujin Kim, The Stony Brook School, NY, USA; Ji Eun Kim, Tabor Academy, MA, USA; Misiakos Panagiotis, Athens College(HAEF), Greece; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; Arber Igrishta, Eqrem Qabej, Vushtrri, Kosovo; Joseph Lee, Loomis Chaffee School in Windsor, CT, USA. Mathematical Reflections 1 (015) 4

5 J38. Let a, b, cbe postive real numbers such that a + b + c =,Prove that a + bc + b + ca + c + ab 3 Proposed by An Zhen-ping, Xianyang Normal University, China and Solution by AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia Without loss of generality, assume that a b c. By the AM-GM inequality, we have a a + bc = + bc a + c (a + c) 1 1 ( a ) + ac + (a + c) a + c ( a + bc a + c = 1 (a + c), b b + ca = + ca b + c (b + c) 1 ( b + ca b + c ) + (a + c) ) + (b + c), c c + ab = + ab b + c (b + c) 1 ( c + ab + (b + c) b + c Summing up the above inequalities and using c b, we have a + bc + b + ca + c + ab 1 ( a + b + 3c + (b + c ) ) + a(b + c) b + c Equality if and only if {a, b, c} = {1, 1, 0}. 1 (3a + b + 3c + b + bc b + c ) = 3 (a + b + c) = 3. Also solved by Yujin Kim, The Stony Brook School, NY, USA; Ji Eun Kim, Tabor Academy, MA, USA; Misiakos Panagiotis, Athens College(HAEF), Greece; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; Myong Claire Yun, Seoul International School, South Korea; Yong Xi Wang,East China Institute Of Technology, China; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Ángel Plaza, University of Las Palmas de Gran Canaria, Spain; Arkady Alt, San Jose, CA, USA; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Francesc Gispert Sanchez, CFIS, Universitat Politecnica de Catalunya, Barcelona, Spain; Jean Heibig, Lycée Stanislas, Paris, France; Joseph Lee, Loomis Chaffee School in Windsor, CT, USA; Nicuşor Zlota, Traian Vuia Technical College, Focşani, Romania; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Titu Zvonaru and Neculai Stanciu, Romania; Utsab Sarkar, Chennai Mathematical Institute, India. ). Mathematical Reflections 1 (015) 5

6 J39. Let a 1, a,..., a 015 be positive integers such that a 1 + a + + a 015 = a 1 a a 015. Prove that among numbers a 1, a,..., a 015 at most nine are greater than 1. Proposed by Titu Zvonaru, Comaneşti and Neculai Stanciu, Buzău, Romania Solution by G.R.A.0 Problem Solving Group, Roma, Italy We show that at most six are greater than 1. Let us assume that a 1 a a d and a d+1 = = a 015 = 1 where d is the maximal number of terms greater than 1. Then 015a 1 a 1 d 1 which implies that d 1 + log (015) = 11. Moreover da d a 1 a d which implies that given a 1 and d 11, then a should satisfies ( d a min(f (a 1, d), a 1 ) where F (a 1, d) := d a 1 ) + d. Note that F is decreasing (not strictly) with respect to a 1 and we can easily solve the problem by considering several cases. 1) d < 11 because F (, 11) = 1. ) d < 10 because F (3, 10) = and F (4, 10) = 1 implies that then a 1 3, a = = a 10 = and a = a a ) d < 9 because F (8, 9) = and F (9, 9) = 1 implies that a 1 8 and by examining all the finite cases we have no solutions. 4) d < 8 because F (16, 8) = and F (17, 8) = 1 implies that a 1 16 and by examining all the finite cases we have no solutions. 5) d < 7 because F (35, 7) = and F (36, 7) = 1 implies that a 1 35 and by examining all the finite cases we have no solutions. 6) d = 6 because F (77, 6) = and F (78, 6) = 1 implies that a 1 77 and by examining all the finite cases we have precisely one solution a 1 = 17, a = 5, a 3 = 3, a 4 = a 5 = a 6 =, a 7 = = a 015 = 1 and = = 040. Also solved by Yujin Kim, The Stony Brook School, NY, USA; Ji Eun Kim, Tabor Academy, MA, USA; Michael Tang, Edina High School, MN, USA; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; Myong Claire Yun, Seoul International School, South Korea; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; George Gavrilopoulos, High School Of Nea Makri, Athens, Greece; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan. Mathematical Reflections 1 (015) 6

7 J330. Let ABCD be a quadrilateral with centroid G, inscribed in a circle with center O, and diagonals intersecting at P. Prove that if O, G, P are collinear, then either ABCD is an isosceles trapezoid or an orthogonal quadrilateral. Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan Let M and N be the midpoints of AC and BD. Observe that G is midpoint of MN and quadrilateral OMNP is cyclic. If AC BD, then OMNP is a rectangle. The diagonals of OMNP bisect each other, so G lies on OP. Assume that ABCD is not orthogonal, then consider Ω, the center of the circumcircle of OM N P. Observe that Ω G and Ω lies on OP. Then ΩM = ΩN, ΩG MN and MO = ON. It follows that triangles AMO is congruent to triangle BNO, so AC = BD. Therefore CDA = BAD and ABCD is an isosceles trapezoid. Also solved by Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 7

8 Senior problems S35. Let S be the incenter of the triangle formed by the midpoints of the triangle ABC. Cevians AS, BS, CS intersect the sides BC, CA, AB in points M, N, P, respectively. Prove that if the perpendiculars to the sides of triangle ABC at points M, N, P are concurrent, then triangle ABC is isosceles. Proposed by Roxana Mihaela Stanciu and Nela Ciceu, Bacău, Romania Solution by Andrea Fanchini, Cantú, Italy The incenter of the triangle DEF formed by the midpoints of the triangle ABC is known as Spieker center and it have barycentric coordinates S = X 10 (b + c : c + a : a + b). So the line AS have equation AS (a + b)y (c + a)z = 0 and then point M = AS }{{} BC, have coordinates x=0 M(0 : c + a : a + b). Line BS have equation BS (a + b)x (b + c)z = 0 and then point N = BS }{{} CA, so it have coordinates N(b + c : 0 : a + y=0 b). Line CS have equation CS (c + a)x (b + c)y = 0 and then point P = CS }{{} AB, so it have coordinates P (b + c : c + a : z=0 0). The infinite point of the perpendicular to the side BC is BC ( a : S C : S B ), so the perpendicular to the side BC at point M have equation MBC [(c + a)s B (a + b)s C ]x a (a + b)y + a (c + a)z = 0 The infinite point of the perpendicular to the side CA is CA (S C : b : S A ), so the perpendicular to the side CA at point N have equation NCA b (a + b)x [(b + c)s A (a + b)s C ]y b (b + c)z = 0 The infinite point of the perpendicular to the side AB is AB (S B : S A : c ), so the perpendicular to the side AB at point P have equation P AB c (c + a)x + c (b + c)y + [(b + c)s A (c + a)s B ]z = 0 Mathematical Reflections 1 (015) 8

9 Now three lines p i x + q i y + r i z = 0, with i = 1,, 3, are concurrent if and only if p 1 q 1 r 1 p q r p 3 q 3 r 3 = 0 In our case the perpendiculars to the sides of triangle ABC at points M, N, P are concurrent if and only if (c + a)s B (a + b)s C a (a + b) a (c + a) b (a + b) (a + b)s C (b + c)s A b (b + c) c (c + a) c (b + c) (b + c)s A (c + a)s B = 0 It s easy to prove that this is true if and only if a = b or b = c or c = a and that s when triangle ABC is isosceles, q.e.d. For example when a = b S A = S B = c, S C = a c and so we have (c + 3a)S A a 3 a 3 a (c + a) a 3 a 3 (c + 3a)S A a (c + a) c (c + a) c (c + a) 0 summing the first two rows we obtain (c + 3a)S A a 3 a 3 a (c + a) (c + 3a)S A (c + 3a)S A 0 c (c + a) c (c + a) 0 = 0 similarly for the other two cases. Also solved by Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan. Mathematical Reflections 1 (015) 9

10 S36. Let a, b, c be positive reals such that, a 3 + b 3 + c 3 + abc = 1 3. Prove that, abc + 9 cyc a 5 4b + bc + 4c 1 4 (a + b + c) (ab + bc + ca) Proposed by Titu Andreescu, University of Texas at Dallas, USA Solution by AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia Applying the Cauchy-Schwarz inequality for the sequences we obtain x 1 = abc, y 1 = 9abc 9a x = 5, y 4b +bc+4c = a(4b + bc + 4c ) 9b x 3 = 5, y 4c +ca+4a 3 = b(4c + ca + 4a ) 9c x 4 = 5, y 4a +ab+4b 4 = c(4a + ab + 4b ) ( a 5 abc + 9 4b + bc + 4c + b 5 4c + ca + 4a + c 5 ) 4a + ab + 4b Equality if and only if a = b = c = 3 (3abc + 3a 3 + 3b 3 + 3c 3 ) 9abc + 4(ab + bc + ca ) + 3abc + 4(ac + ba + cb ) 1 = 4[(ab + abc + ba ) + (bc + abc + c b) + (ca + abc + ac )] 1 = 4(a + b + c)(ab + bc + ca) Also solved by Utsab Sarkar, Chennai Mathematical Institute, India; Arkady Alt, San Jose, CA, USA; Behzod Ibodullaev, Lyceum TCTU, Tashkent, Uzbekistan; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Nicuşor Zlota, Traian Vuia Technical College, Focşani, Romania; Odilbek Utamuratov, Uzbekistan; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 10

11 S37. Let H be the orthocenter of an acute triangle ABC and let D and E be the feet of the altitudes from vertices B and C, respectively. Line DE intersects the circumcircle of triangle ABC in points F and G (F lies on the smaller arc of AB and G lies on the smaller arc of AC). Denote by S the projection of H onto line DE. Prove that EF + DS = DG + ES. Proposed by İlker Can Çiçek, Instanbul, Turkey First solution by Prithwijit De, HBCSE, Mumbai, India Assume B C. Let HS = x and DE = y. Angle chasing gives us DHS = B and EHS = C. Also x = R cos A cos B cos C, where R is the circumradius of triangle ABC and y = a cos A. Thus DS ES = x(tan B tan C) = R cos A sin(b C). (1) By the intersecting chord theorem it follows that DG DF = AD DC and EG EF = AE EB. Subtracting one from the other leads to DG EF = AD DC AE EB y = a cos A(c cos C b cos B) a cos A = R cos A sin(b C). () Hence DS ES = DG EF and the result follows. Second solution by Ivan Borsenco, Massachusetts Institute of Technology, USA Let M be the midpoint of F G and N be the midpoint of DE. Denote by O and Ω the circumcenter and the center of the nine-point circle of triangle ABC. Observe that M is the projection of O, N is the projection of Ω, and S is the projection of H onto F G. Because OΩ = HΩ, we have MN = NS, DS = EM and ES = DM. From the fact that F M = MG, we have EF + EM = DG + DM, which is equivalent to EF + DS = DG + ES, as desired. Also solved by Misiakos Panagiotis, Athens College(HAEF), Greece; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; George Gavrilopoulos, High School Of Nea Makri, Athens, Greece; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Andrea Fanchini, Cantú, Italy; Arkady Alt, San Jose, CA, USA; Behzod Ibodullaev, Lyceum TCTU, Tashkent, Uzbekistan; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Saturnino Campo Ruiz, Salamanca, Spain; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 11

12 S38. Let k be an odd positive integer. There are k positive integers written on a circle that add up to k. Prove that for any 1 m k, among the k given numbers, we can find one or more consecutively placed numbers that add up to m. Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by the author Let a 1,..., a k be numbers written on a circle. Consider k sums of m consecutive numbers on a circle: s i = a i + a i a i+m 1, where i {1,..., k} and the indices are taken modulo k. We have s 1 + s + + s k = m(a 1 + a + + a k ) = 4km. If all s i are equal to 4m, then a i = a m+i for all i {1,..., k}. Observe that because k is odd, we have d = gcd(k, m) m. We find that in this case our k numbers on a circle, consist of k d congruent blocks of d numbers. The sum of numbers in each block is d, so if we consider m d consecutive blocks, their sum is m, as desired. If not all s i are equal, then there exists s j such that s j 4m 1. Consider m sums: a j, a j + a j+1,..., a j + a j a j+m 1. If any of these sums is equal to m, we are done. Otherwise, each of these sums is less than 4m and by the pigeonhole principle, two of these sums give the same residue module m. Taking their difference, we find a sum of several consecutive numbers on circle that is equal to m. Mathematical Reflections 1 (015) 1

13 S39. Given a quadrilateral ABCD with AB + AD = BC + DC let (I 1 ) be the incircle of triangle ABC which is tangent to AC at E, and let (I ) be the incircle of triangle ADC which is tangent to AC at F. The diagonals AC and BD intersect at point P. Suppose BI 1 and DE intersect at point S and DI and BF intersect at point T. Prove that S, P, T are collinear. Proposed by Khakimboy Egamberganov, Tashkent, Uzbekistan Solution by Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan At first we use these following lemmas: Lemma 1 : Let ABCD be a quadrilateral. Prove that AB + AD = BC + BD if and only if ABCD is B extangential or D extangential. Proof: Assume that ABCD is B or D extangential. It is enough to prove in case of ABCD is B extangential. Let B excircle of ABCD tangents to the sides BA, CD, AD, BC at points X, Y, Z, T, respectively. We have BX = BT or BA + AD + DZ = BC + CD + DY and DZ = DY, so BA + AD = BC + CD Assume that BA + AD = BC + CD (1) and let Ω be the circle tangent to lines BA, CD, AD. It is enough to prove that CB is tangent to Ω.Let tangent line from C to Ω intersects BA at B 0 Then quadrilateral AB 0 CD - B 0 - extangencial. = AB 0 + AD = CD + CD 0 (). We have from (1) and () that B = B 0, hence Lemma1 is proved. Lemma : The lines I 1 I, AC, BD are concurrent. Proof : From Lemma 1 ABCD is B extangential. Let I 1 I AC = P 0 is insimilicenter of (I 1 ) and (I ). However B is exsimilicenter of (I 1 ) and Ω, and D is insimilicenter of (I ) and Ω. From Monge-D Alembert cirle theorem we have B, D, P 0 are collinear, so P 0 = P. Lemma 3 : Let Γ b is B excircle of ABC and Γ d be is D excircle of ADC. Then AC is tangent to Γ b at F and is also tangent to Γ d at E. Proof:It is obviously, we have CA+CB AB = AD+AC DC. Hence F is exsimilicenter of (I ) and Γ b, E is exsimilicenter of (I 1 ) and Γ d. We prove the problem by applying abovementioned lemmas: Let T 0 be the exsimilicenter of (I ) and Ω. We have by Monge-D Alembert cirle theorem: (1) For {(I 1 ), (I ), Γ b } we obtain T 0, F, B are collinear, so T 0 BF. () For {(I ), Ω, Γ b } we get T 0, I, I are collinear, so T 0 DI. From (1) and () we have T = T 0 or T is exsimilicenter of (I ) and Ω. Similarly we have S is insimilicenter of (I 1 ) and Ω. Finally applying by Monge-D Alembert cirle theorem for (I 1 ), (I ), Ω we obtain that T, S, P are collinear. Mathematical Reflections 1 (015) 13

14 S330. Let x 1, x,..., x n be real numbers, n, such that x 1 + x + + x n = 1. Prove that, Where k n = n 1. n 1 x i + k n x j x k n 1 i=1 1 j<k n Proposed by Marius Stănean, Zalău, Romania Solution by the author We have two cases: 1. if x j x k 0 then. if 1 j<k n 1 j<k n n i=1 1 x i + k n x j x k 0 then noting q = By squaring both sides we get n (1 x i ) + i=1 1 j<k n 1 j<k n 1 x j n i=1 1 j<k n 1 j<k n x j x k n 1 x i = n 1; i=1 x j x k 0 the inequality we can write as 1 x i n 1 k n q. 1 x j 1 x k (n 1) (n 1)k n q + knq 1 x k (n 1)(n ) (n 1)k nq + k nq. Using Cauchy-Schwarz inequality we have for 1 j < k n 1 x j 1 x k = x x j 1 + x j x n x x k 1 + x k x n x x j 1 + x j x k 1 + x k x n + x j x k and summing this for all j, k such that 1 j < k n we get 1 x j 1 x k 1 x i x k + x jx k 1 j<k n 1 j<k n ( ) n (n 1)(x 1 + x x n) + q Therefore to prove the inequality (1) it is enough to show that = (n 1)(n ) + q (n 1)(n ) + q (n 1)(n ) (n 1)k n q + k nq q( + (n 1)k n k nq) 0. But k n is a root of the quadratic equation x 4x 4 n 1 = 0 (n 1)x 4(n 1)x 4 = 0, so +(n 1)k n k n = n 1 therefore () is equivalent with the following inequality ( ) n 1 q q 0 Mathematical Reflections 1 (015) 14

15 which is true because q 0 and q n 1. The last inequality follows from (x j x k ) 0 (n 1)(x 1 + x x n) 1 j<k n 1 j<k n The equality hold when q = 0 which means that one of the numbers x 1, x,..., x n is ±1 and the other 0 or q = n 1 1 which means that x 1 = x =... = x n = ± n. Also solved by Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan, Utsab Sarkar, Chennai Mathematical Institute, India x j x k. Mathematical Reflections 1 (015) 15

16 Undergraduate problems U35. Let A 1 B 1 C 1 be a triangle with circumradius R 1. For each n 1, the incircle of triangle A n B n C n is tangent to its sides at points A n+1, B n+1, C n+1. The circumradius of triangle A n+1 B n+1 C n+1, which is also the inradius of triangle A n B n C n is R n+1. Find lim n R n+1 R n. Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by Yong Xi Wang,East China Institute Of Technology, China Suppose A n B n C n has an incircle with radius r n,then we have R n+1 = r n and use well known on the other hand,we have r n = 4 sin A n R n sin B n sin C n A n+1 = π A n, B n+1 = π B n C n+1 = π C n Because A n + B n+1 IC n+1 = π, C n+1 A n+1 B n+1 = B n+1 IC n+1 so we have simaler we have so we have so A n+1 π 3 = 1 ( A n π ) = A n = π ( A 1 π ) ( 1 n 1 3 ) B n = π ( 3 + B 1 π ) ( 1 ) n 1, C n = π ( C π ) ( 1 ) n 1 3 R n+1 lim n R n lim A n = lim B n = lim C n = π n n n 3 r n = lim = lim n R 4 sin π n n 6 sin π 6 sin π 6 = 1 Also solved by Michael Tang, Edina High School, MN, USA; Ji Eun Kim, Tabor Academy, MA, USA; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Arkady Alt, San Jose, CA, USA; Saturnino Campo Ruiz, Salamanca, Spain; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 16

17 U36. Find n=0 a n + a n + a n + 1 where a 0 > 1 and 3a n+1 = a 3 n + for all integers n 1. Proposed by Arkady Alt, San Jose, CA, USA Solution by G. C. Greubel, Newport News, VA, USA Consider the series n=0 a n + a n + a n + 1 where 3a n+1 = a 3 n +. The reduction of the series is as follows. where 3a n+1 = a 3 n + was used. Now, S = = = = S = = n=0 n=0 n=0 a n + a n + a n + 1 (a n 1)(a n + ) a 3 n 1 a n + a n 3(a n=0 n+1 1) (a n 1)(a n + a n ) 3(a n=0 n 1)(a n+1 1) a n+1 a n (a n=0 n 1)(a n+1 1) ( ) 1 a n 1 1. a n+1 1 The collapse of this telescopic series leads to the desired result, namely, n=0 a n + a n + a n + 1 = 1 a 0 1. Also solved by Ji Eun Kim, Tabor Academy, MA, USA; Michael Tang, Edina High School, MN, USA; Kyoung A Lee, The Hotchkiss School, Lakeville, CT, USA; AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Yong Xi Wang,East China Institute Of Technology, China; Brian Bradie, Christopher Newport University, Newport News, VA, USA; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Cailan Li; Haroun Meghaichi, University of Science and Technology Houari Boumediene, Algiers, Algeria; Moubinool Omarjee Lycée Henri IV, Paris, France; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Prithwijit De, HBCSE, Mumbai, India; Albert Stadler, Herrliberg, Switzerland; Alessandro Ventullo, Milan, Italy. Mathematical Reflections 1 (015) 17

18 U37. Let (a n ) n 0 be a sequence of real numbers with a 0 = 1 and Find the limit lim n n3 a n. a n+1 = a n n a n + a n + 1. Proposed by Khakimboy Egamberganov, Tashkent, Uzbekistan Solution by Alessandro Ventullo, Milan, Italy Observe that a n+1 < a n n = 1 a n n, so n=0 a n converges by the Comparison Test. Let a n = c R. n=0 We have So, 1 = n + a n + 1. a n+1 a n 1 a 1 = 0 + a a 0 1 a = 1 + a a 1 and summing the two columns, we get... 1 = (n 1) + a n 1 + 1, a n a n 1 n 1 1 (n 1)n(n 1) = + a k + 1. a n 6 k=0 Hence, Since we get so 1 (n 1)n(n 1) n 1 k=0 n 3 = a n 6n 3 + a k + 1 n 3. n 1 k=0 lim a k + 1 c + 1 n n 3 = lim n n 3 = 0, lim n 1 (n 1)n(n 1) n 3 = lim a n n 6n 3 = 1 3, lim n n3 a n = 3. Also solved by Arkady Alt, San Jose, CA, USA; Ji Eun Kim, Tabor Academy, MA, USA; AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Yong Xi Wang,East China Institute Of Technology, China; Brian Bradie, Christopher Newport University, Newport News, VA, USA; Haroun Meghaichi, University of Science and Technology Houari Boumediene, Algiers, Algeria; Moubinool Omarjee Lycée Henri IV, Paris, France; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Albert Stadler, Herrliberg, Switzerland. Mathematical Reflections 1 (015) 18

19 U38. Let (a n ) n 1 be an increasing sequence of positive real numbers such that lim n a n = and the sequence (a n+1 a n ) n 1 is monotonic. Evaluate a a n lim n n a n Proposed by Mihai Piticari and Sorin Rădulescu, Romania Solution by Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy The sequence (a n+1 a n ) n 1 converges to a limit L because of its monotonicity. Suppose L > 0 is finite. By Cesaro Stolz we have L = lim (a a n n+1 a n ) = L = lim n n n Moreover lim ( a n+1 a n+1 a n a n ) = lim n n an+1 + = L a n = 0 Also by Cesaro Stolz we consider The reciprocal gives and (a a n+1 ) (a a n ) lim n (n + 1) a n+1 n = lim a n n (n + 1) a n+1 n a n lim n a n+1 a n+1 (n + 1) a n+1 n a n ( n + 1 = lim ( a n+1 ) an a n ) + n a n+1 a n+1 The conclusion is n + 1 lim ( a n+1 a n ) = 1 n a n+1 L 0 = 0, lim an = 0 n a n+1 a n+1 lim n a n+1 (n + 1) a n+1 n a n = because (n + 1) a n+1 n is positive via the increasing monotonicity of (a n ) and a n > 0. a n Now let L = lim (a a n n+1 a n ) = 0 and then 0 = lim n n n by Cesaro Stolz. As above we consider that is (a a n+1 ) (a a n ) lim n (n + 1) a n+1 n = lim a n n a n+1 (n + 1) a n+1 n a n lim n Now we prove that an+1 a n n is bounded above. We write an+1 an+1 ( n a n+1 an+1 a n an n a n+1 a n an+1 an+1 + n(a n+1 a n ) a n a n ) Mathematical Reflections 1 (015) 19

20 Lemma: Under the assumptions of the statement of the problem, the positive sequence n(a n+1 a n ) is a n bounded above. Proof: Let s argue by contradiction supposing that for any positive p there exists n p such that n P (a np+1 a np ) or a np+1 a np pa n p. The monotonicity of a np n p a n+1 a n yields This implies a n+1 a n a np+1 a np pa n p n p 1 n n p 1 n p pa np a 1 = a np + (a k a k+1 ) < a np n p = n p (1 p) < 0 n p k=1 and this is a contradiction with the positivity of the sequence {a n }. The consequence of the Lemma is the existence of a constant C such that 0 < n(a n+1 a n ) a n implies lim n an+1 ( n a n+1 an+1 a n an ) a n+1 an+1 (C + 1) = The last step is the case L =. This means that for all p > 0 there exists n p such that n > n p implies a n+1 a n > p. It follows C. This n p 1 a n = a 1 + (a k+1 a k ) + k=1 n k=n p (a k+1 a k ) a 1 + (n p 1) min 1 k n p 1 (a k+1 a k ) + p(n n p + 1) = a 1 + (n p 1)(a a 1 ) + p(n n p + 1) p(n n p + 1) p n if n (n p 1) The monotonicity increasing of {a n } has been used. Based on this we can write a a np 3 + ( a np a n ) (np 3)a 1 + (n n p + 3)p n (n n p + 3)p n pn 4 for any n 4n p + 6. Finally we put the above lover bounds in a a n n and write a n a a n n a a n a a n a a n = p n 1 a n n an n n = p Since p can be as large as we want, this shows that also when L =. a a n lim n n a n = Also solved by Moubinool Omarjee Lycée Henri IV, Paris, France. Mathematical Reflections 1 (015) 0

21 U39. Let a 1 a... a n(n 1) be the distances between n distinct points lying on the plane. Prove that there is a constant c such that for any n we can find indices i and j such that a i 1 a j < c ln n n. Proposed by Nairi Sedrakyan, Yerevan, Armenia Solution by Ivan Borsenco, Massachusetts Institute of Technology, USA Assume that for any pair of indices (i, j) we have a i a j 1 d and d is the greatest such number. Then a n(n 1) n(n 1) (1 + d)a n(n 1)... (1 + d) 1 a 1 1. Let AB = a n(n 1) and CD = a 1. From the fact that AC + BC > AB, we can assume without loss of generality that AC AB. Then and d(1 + d) n(n 1) 1. d min(ac, AD) max(ac, AD) 1 = CD max(ac, AD) CD AC CD AB = If in our configuration AB = a n(n 1) and AC = a 1, then d AC AB 1 BC AB = 1 (1 + d) n(n 1) In any case, we obtain d(1 + d) n(n 1) Assume that n 4 and consider f(x) = x(1 + x) n(n 1) 1. Observe that (1 + d) n(n 1) 1 Because ( ) 100 ln n f = n ( 100 ln n n ( n 100 ln n ) n 100 ln n +1 e, ) n(n 1) 1. we get Hence ( ) 100 ln n f n 100 ln n n n 100 ln n n 100 ln n Therefore for c = 100, we always have d < c ln n n. e 100 ln n n +100 ln n > e 10 ln n n. ( ) e 10 ln n n(n 1) n ln n n e 3 ln n = 100 ln n n >. Mathematical Reflections 1 (015) 1

22 U330. Find all functions f : R R with the properties (i) f is an antiderivative (ii) f is integrable on any compact interval (iii) f (x) = x 0 f(t)dt for all x R Proposed by Mihai Piticari, Câmpulung Moldovenesc, Romania Solution by Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy We claim that f 1 (x) 0 and f (x) = x/ are solutions. We are going to prove that there are no other solutions. Clearly f(0) = 0. If f is an antiderivative, it is differentiable so by (iii) f(x)f (x) = f(x) It follows that if f(x 0 ) 0 then f (x 0 ) = 1/. Suppose now that there exists a function g(x) satisfying (i) (iii) but g(x) f 1 and g f. This means that there exists a point ξ > 0 (or ξ < 0) such that g(ξ) 0 and g(ξ) ξ/. By the mean value theorem g(ξ) g(0) ξ 0 = g (η), If g(η) 0 we have g (η) = 1/ and then g(ξ) = ξ/ which is impossible so we are forced to sat that g(η) = 0. Now, consider the following two Lemmas. Lemma 1: If f satisfying (i) (iii) is such that f(x 1 ) = 0, x 1 0, (say x 1 > 0) then f(x) = 0 for any x [0, x 1 ]. Proof: Suppose f(x ) 0, 0 < x < x 1. By the mean value theorem it would follow that a value x 3 would exist such that f(x 3 ) 0 and f (x 3 ) < 0. but this is impossible. Lemma : If f satisfies (i) (iii) and f(x) 0, for x [a, b] with a 0 b, a b, the f(x) = 0 for any x R. Proof: Suppose indeed f(x ) 0 and x > b. By f (x ) = 1/ and the fact that any derivative is a "Darboux function", it follows that f (x) assumes all the value between the value 0 and 1/ but this is impossible. The conclusion is that the function g(x) described above, does not exist. Mathematical Reflections 1 (015)

23 Olympiad problems O35. The taxicab distance between points P 1 = (x 1, y 1 ) and P (x, y ) in a coordinate plane is given by d(p 1 P ) = x 1 x + y 1 y. The taxicab disk with center O and radius R is the set of points P such that d(p, O) R. Given n points that are pairwise at most R taxicab distance apart, find the smallest constant c such that any such set of points can be covered by a taxicab disk of radius cr. Proposed by Ivan Borsenco, Massachusetts Institute of Technology, USA Solution by the author Assume that points are at most a unit taxicab distance apart. We are looking for the smallest c such that any such set of points can be covered by the taxicab disk of radius c. Consider four points with coordinates (± 1 4, ± 1 4 ). Then they are at most one taxicab unit apart and only can be covered by a taxicab disk with radius at least 1. We prove that c = 1 is the smallest constant. Consider a rotation of the plane with center at the ( origin and angle 45. Any ) taxicab disk becomes a square and points P i = (x i, y i ) become P i = (x i, y i ) = 1 1 (x i + y i ), (x i y i ). We are looking for the least square that can cover our rotated set. Note that x-coordinates of any two points in the rotated set are at most 1 apart: 1 x i + y i x j y j 1 ( x i x j + y i y j ) 1. Similarly, any two y-coordinates are at most 1 apart. We have max(x i ) min(x i ) 1 and max(y i ) min(y i ) 1. Thus rotated set can be covered by a 1 1 square, which initially corresponds to a taxicab disk of radius 1. Mathematical Reflections 1 (015) 3

24 O36. Let ABC be a non-isosceles triangle. Let D be a point on the ray AB, but not on the segment AB, and let E be a point on the ray AC, but not on the segment AC, such that AB BD = AC CE. The circumcircles of triangles ABE and ACD intersect in points A and F. Let O 1 and O be the circumcenters of triangles ABC and ADE. Prove that lines AF, O 1 O, and BC are concurrent. Proposed by İlker Can Çiçek, Instanbul, Turkey Solution by Adnan Ali, Student in A.E.C.S-4, Mumbai, India We first prove two Lemmas: Lemma 1: O 1 O is the perpendicular bisector of BC. Proof: It is sufficient to show that O lies on the perpendicular bisector of BC. Let r be the radius of (ADE). Then, power of B, C w.r.t to (ADE) is expressed as AB BD = r O B = AC CE = r O C Thus O B = O C, proving that O lies on the perpendicular bisector of BC. Lemma : AF bisects BC. Proof: Let M = AF BC, BC (ABE) = Q, CB (ADC) = P. Then, power of C w.r.t to (ABE) and power of B w.r.t (ADC) can be expressed as respectively: thus showing that P B = QC (1). Now expressing the power of M w.r.t to (ABE) and power of M w.r.t to (ADC) as Hence Hence AF bisects BC. AC CE = QC CB = AB BD = P B BC AM MF = QM MB AM MF = P M MC. P M MC = QM MB P B MC + BM MC = QC MB + MB MC P B MC = QC MB MB = MC (from (1)). Now from Lemma 1 and Lemma we conclude that AF, O 1 O, BC concur at M, the midpoint of BC. Also solved by Misiakos Panagiotis, Athens College(HAEF), Greece; Andrea Fanchini, Cantú, Italy; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 4

25 O37. Let a, b, c be positive reals such that a + b + c + abc = 4. Prove that, a + b + c a + b + c Proposed by An Zhen-ping, Xianyang Normal University, China Solution by Utsab Sarkar, Chennai Mathematical Institute, India Notice that a, b, c (0, ) and hence we ll make the substitution for a = cos A, b = cos B, c = cos C. Therefore our constraint becomes to, cos A + cos B + cos C + cos A cos B cos C = 1 = A + B + C = π Thus for a triangle ABC we need only to prove, ( ) a a 0 (sin A ) cos A 0 cyc cyc cyc (a + b c)(a + c b) bc cyc b + c a bc cyc a bc(a + b c)(a + c b) cyc (a b+a c a 3 ) Now using Ravi Transformation we see, a = y +z, b = z +x, c = x+y which provides an equivalent inequality to prove, (y + z) x Where x, y, z are positive reals. By Cauchy Schwarz Inequality we get, Now,it only remains to show, cyc (y + z) (x + y)(x + z)yz cyc (x + y)(x + z) (x + yz) cyc (y + z) yz (x + yz) cyc (y + z) x cyc x(y + z) yz 6xyz And the last inequality is true by AM-GM, since x(y + z) yz x yz yz = xyz = 6xyz cyc cyc cyc. Also solved by Ji Eun Kim, Tabor Academy, MA, USA; Misiakos Panagiotis, Athens College(HAEF), Greece; AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Yong Xi Wang,East China Institute Of Technology, China; Adnan Ali, Student in A.E.C.S-4, Mumbai, India; Arkady Alt, San Jose, CA, USA; Behzod Ibodullaev, Lyceum TCTU, Tashkent, Uzbekistan; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Nicuşor Zlota, Traian Vuia Technical College, Focşani, Romania; Paolo Perfetti, Universitá degli studi di Tor Vergata Roma, Roma, Italy; Piriyakorn, Triam Udom Suksa School, Thailand; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Shatlyk Mamedov, School Nr. 1, Dashoguz, Turkmenistan; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 5

26 O38. The diagonals AD, BE, CF of the convex hexagon ABCDEF intersect at point M. Triangles ABM, BCM, CDM, DEM, EF M, F AM are acute. Prove that circumcenters of these triangles are concyclic if and only if the areas of quadrilaterals ABDE, BCEF, CDF A are equal. Proposed by Nairi Sedrakyan, Yerevan, Armenia Solution by the author Proof of necessity: Let O 1, O, O 3, O 4, O 5 and O 6 be the centers of the circumcircles of triangles ABM, BCM, CDM, DEM, EF M and F AM, respectively. Notice that O 1, O and O 4, O 5 are laying on the perpendicular bisectors of BM and EM, respectively. Therefore, O 1 O BE and O 4 O 5 BE O 1 O O 4 O 5. Moreover, the projection of O 1 O 4 onto BE has length 1 BE. Since O 1, O, O 4, O 5 are concyclic by condition of the problem and O 1 O O 4 O 5 O 1 O 4 = O O 5, and BE forms equal angles with O 1 O 4 and O O 5. By analogy we get O 1 O 4 = O 3 O 6. Consider T 1 T 4, T T 5, T 3 T 6 sharing the same midpoint and T 1 T 4 = O 1 O 4, T T 5 = O O 5, T 3 T 6 = O 3 O 6. Note that T 1 T T 4 T 5 is a rectangle, therefore, T 1 T 5 BE and T 1 T 5 is the projection of T 1 T 4 onto T 1 T 5 T 1 T 5 = 1 BE. Similarly we get T 1 T 3 AD, T 1 T 3 = 1 AD. Finally, S ABDE = 1 AD BE sin AMB = 1 T 1T 3 T 1 T 5 sin T 3 T 1 T 5 = 4S T1 T 3 T 5. Proof of sufficiency: Again, let O 1, O, O 3, O 4, O 5 and O 6 be the centers of the circumcircles of triangles ABM, BCM, CDM, DEM, EF M and F AM, respectively. Notice that O 1, O and O 4, O 5 are laying on the perpendicular bisectors of BM and EM, respectively. Therefore, O 1 O BE and O 4 O 5 BE O 1 O O 4 O 5. Let AD = a, BE = b, CF = c, AMB = γ, BMC = α, CMD = β. Since α + β + γ = 180 o and S ABDE = ab sin γ, S BCEF = bs sin α, S CDF A = ac sin β S ABDE = S BCEF = S CDF A sin α a = sin β b = sin γ. c We deduce that α, β, γ are angles of a triangle with sides a, b and c. The lengths of projections of O 1 O 4 onto BE and AD are equal to 1 BE and 1 AD, respectively. Mathematical Reflections 1 (015) 6

27 Hence, O 1 O 4 = R, where R - radius of the circumcircle of the triagle with sidelengths a, b, c. and By analogy we get that O O 5 = R, O 3 O 6 = R O 1 O 4 = O O 5 = O 3 O 6. Finally, O 1 O O 4 O 5, O O 3 O 5 O 6, O 3 O 4 O 6 O 1 O 1 O 4 = O O 5 = O 3 O 6 yields that by condition of the problem, O 1, O, O 3, O 4, O 5 and O 6 are concyclic. Also solved by Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan. Mathematical Reflections 1 (015) 7

28 O39. For any positive integer r, denote by P r the path on r vertices. For any positive integer g, prove that there exists a graph G with no cycles of length less than g with the following property: any two coloring of the vertices of G contains a monochromatic copy of P r. Solution by the author Proposed by Cosmin Pohoata, Columbia University, USA Recall the celebrated result by Erdös that there exists graphs with arbitrarily large girth and arbitrarily large chromatic number. More precisely, for any positive integers k and g, there is some graph G which has no cycle of length less than g and for which χ(g) k. This was one of the first applications of the probabilistic method in combinatorics. For the proof, see for example Lecture 11 from Daniel Spielman s graph theory class at Yale: For our problem, we choose k = 4r + 1. Next, notice that if some vertex x V (G) has degree < 4r, then we can use induction to argue that we can in fact color G using 4r colors. Indeed, remove x from G and notice that any 4n-coloring of the new graph can be augmented to a coloring of G (since we can assign x one of the 4n colors, provided that deg x < 4r). This contradicts the fact that χ(g) 4r + 1. Consequently, every graph G with chromatic number at least 4n + 1 has a subgraph with a minimum degree vertex at least 4r. It follows that any two coloring of G contains a monochromatic subgraph of minimum degree at least r. Within this monochromatic subgraph, it is easy to see that we have a path of length r; hence we found a monochromatic copy of P r. Mathematical Reflections 1 (015) 8

29 O330. Four segments divide a convex quadrilateral into nine quadrilaterals. The points of intersections of these segments lie on the diagonals of the quadrilateral (see figure below). It is known that quadrilaterals Q 1, Q, Q 3, Q 4 admit inscribed circles. Prove that the quadrilateral Q 5 also has an inscribed circle. Proposed by Nairi Sedrakyan, Yerevan, Armenia Solution by Cosmin Pohoata, Columbia University, USA The following is an exposition of the proof from Cosmin Pohoata and Titu Andreescu, 110 Geometry Problems for the International Mathematical Olympiad, XYZ Press, 014. The key idea is to make use of the following result due to Marius Iosifescu repeatedly. Iosifescu s Theorem. A convex quadrilateral ABCD admits an inscribed circle if and only if tan x tan z = tan y tan w, where x = ABD, y = ADB, z = BDC and w = DBC. Proof. Using the trigonometric formula tan u = 1 cos u 1 + cos u, we get that the equality in the theory is equivalent to so that and (1 cos x)(1 cos z)(1 + cos y)(1 + cos w) = (1 cos y)(1 cos w)(1 + cos x)(1 + cos z). Let a = AB, b = BC, c = CD, d = DA and q = BD. From the Law of Cosines, we have In the same way 1 cos y = 1 cos x = d (a q) aq 1 + cos x = (a + q) d aq cos x = a + q d, aq (a + d q)(a d + q), 1 + cos y = dq = = (d + a q)(d a + q) aq (a + q + d)(a + q d). aq (d + q + a)(d + q a), dq Mathematical Reflections 1 (015) 9

30 where 1 cos z = 1 cos w = (b + c q)(b c + q), 1 + cos z = cq (c + b q)(c b + q), 1 + cos w = bq Thus, after some rearranging, the identity we started with becomes (c + q + b)(c + q b), cq (b + q + c)(b + q c). bq P ( (d a + q) (b c + q) (a d + q) (c b + q) ) = 0, P = d + a q)(b + c q)(d + q + a)(b + q + c) 16abcdq 4 is a positive expression according to the triangle inequality applied in triangles ABD and BCD. Factoring the above and cancelling out terms in the parantheses yields 4qP (b + d a c) ( (d a)(b c) + q ) = 0, where the expression in the second paranthesis can never be equal to zero - since, by the triangle inequality, q > a d and q > b c, so q is always either (strictly) less or greater than (a d)(b c). It thus follows that tan x tan z = tan y tan w holds if and only if b + d a c = 0, i.e. b + d = a + c. By Pithot s theorem, we therefore conclude that this happens if and only if ABCD admits an inscribed circle. In order to use Iosifescu s theorem for quadrilateral 5, we aim to prove that tan F HG tan BDC = tan ADB tan EHF. However, quadrilateral 3 has an incircle, so It thus suffices to show that tan EF H tan EF H tan F HG tan ADB = tan EHF = tan HF G tan HF G. tan BDC. However, by applying Iosifescu s lemma once again to quadrilateral 1, the above happens if and only if tan ABD tan BDC = tan ADB tan DBC, which we know is true if and only if ABCD has inscribed circle. This is where the magic comes in. We show that ABCD has an inscribed circle by using Iosifescu s theorem for the other diagonal AC of ABCD. Namely, it suffices to show that tan BAC tan ACD = tan BCA tan CAD. This is however easy to prove in the same style as above. Indeed, by writing out Iosifescu s lemma for quadrilaterals, 3 and 4, and keeping in mind the vertical angles from the vertices of quadrilateral 3, we get precisely the above statement. We leave the details for the reader. Also solved by Adnan Ali, Mumbai, India, AN-anduud Problem Solving Group, Ulaanbaatar, Mongolia; Farrukh Mukhammadiev, Academic Lyceum Nr.1 under the SamIES, Samarkand, Uzbekistan; Sardor Bozorboyev, Lyceum S.H.Sirojjidinov, Tashkent, Uzbekistan; Shatlyk Mamedov, School Nr. 1, Dashoguz, Turkmenistan; Titu Zvonaru and Neculai Stanciu, Romania. Mathematical Reflections 1 (015) 30