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- Geometry Problems in RMO
- Number Theory Problems in RMO
- Algebra (Complex numbers, inequality, theory of equation and functional equation) Problems in RMO

### RMO 1991

- Let P be an interior point of triangle ABC and AP, BP, CP meet the sides BC, CA, AB in D, E, F respectively. Show that \( \frac{AP}{PD} = \frac{AF}{FB} +\frac{AE}{EC} \).
- If a, b, c and d are any four positive real numbers, then prove that \(\frac{a}{b}+\frac{b}{c}+\frac{c}{d}+\frac{d}{a} \ge 4 \).
- A four-digit number has the following properties:
- it is a perfect square;
- its first two digits are equal to each other;
- its last two digits are equal to each other; Find all such four digit numbers.

- There are two Urns each containing an arbitrary number of balls (both are non empty to begin with). We are allowed two types of operations:
- remove an equal number of balls simultaneously from both the urns and
- double the number of balls in any one of them.

Show that after performing these operations finitely many times , both the urns can be made empty.

- Take any point \(P_{1} \) on the side BC of a triangle ABC and draw the following chain of lines: \(P_{1}P_{2} \) parallel to AC, \(P_{2}P_{3} \) parallel to BC, \(P_{3}P_{4} \) parallel to BC, \(P_{4}P_{5} \) parallel to CA, and \(P_{4}P_{5} \) parallel to BC. Here lie on AB \(P_{3}P_{4} \) on CA and \(P_{4} \) on BC. Show that \(P_{6}P_{1} \) is parallel to AB.
- Find all integer values of a such that the quadratic expression (x+a)(x+1991) + 1 can be factored as a product (x+b)(x+c) where b and c are integers.
- Prove that \(n^{4}+4^{n} \) is composite for all integer values of n > 1.
- The 64 squares of a \(8 times 8 \) chessboard are filled with positive integers in such a way that each integer is the average of the integers on the neighbouring squares. (Two squares are neighbours if they share a common edge or a vertex. Thus a square can have 8, 5 or 3 neighbours depending on its position).Show that all the 64 integers entries are in fact equal.

### RMO 1992

- Determine the set of integers n for which \(n^2 +19n + 92 \) is a square of an integer.
- If \(\frac{1}{a}+\frac{1}{b}=\frac{1}{c} \) where a, b, c are positive integers with no common factor, prove that (a + b) is the square of an integer.
- Determine the largest 3-digit prime factor of the integer \({2000} \choose {1000} \) .
- ABCD is a cyclic quadrilateral with AC perpendicular to BD; AC meets BD at E. Prove that \(R \) is the radius of the circumscribing circle.
- ABCD is a cyclic quadrilateral; x, y, z are the distances of A from the lines BD, BC, CD respectively. Prove that \(\frac{BD}{x}=\frac{BC}{y}+\frac{CD}{z} \)
- ABCD is a quadrilateral and P, Q are mid-points of CD, AB respectively. Let AP, DQ meet at X, and BP, CQ meet at Y . Prove that area of ADX + area of BCY = area of quadrilateral PXQY .
- Prove that \(1 < \frac{1}{1001} +\frac{1}{1002}+\frac{1}{1003}+ ……………+\frac{1}{3001} < \frac{4}{3} \)
- Solve the system (x + y)(x + y + z) = 18, (y + z)(x + y + z) = 30, (z + x)(x + y + z) = 2A in terms of the parameter A.
- The cyclic octagon ABCDEFGH has sides a, a, a, a, b, b, b, b respectively. Find the radius of the circle that circumscribes ABCDEFGH in terms of a and b.

### RMO 1993

- Let ABC be an acute-angled triangle and CD be the altitude through C. If AB = 8 and CD = 6, find the distance between the mid-points of AD and BC.
- Prove that the ten’s digit of any power of 3 is even. [e.g. the ten’s digit of \(3^{6} \)= 729 is 2]
- Suppose \(A_1A_2A_3…..A_n \) is a 20-sided regular polygon. How many non-isosceles (scalene) triangles can be formed whose vertices are among the vertices of the polygon but whose sides are not the sides of the polygon?
- Let ABCD be a rectangle with AB = a and BC = b. Suppose \(r_1 \) is the radius of the circle passing through A and B and touching CD; and similarly \(r_2 \) is the radius of the circle passing through B and C and touching AD. Show that \(r_1 +r_2\ge\frac{5}{8}(a+b) \).
- Show that \(19^{93}+13^{99} \) is a positive integer divisible by 162.
- If a, b, c, d are four positive real numbers such that abcd = 1, prove that \((1+a)(1+b)(1+c)(1+d)\ge16 \).
- In a group of ten persons, each person is asked to write the sum of the ages of all the other 9 persons. If all the ten sums form the 9-element set {82, 83, 84, 85, 87, 89, 90, 91, 92} find the individual ages of the persons (assuming them to be whole numbers of years).
- I have 6 friends and during a vacation I met them during several dinners. I found that I dined with all the 6 exactly on 1 day; with every 5 of them on 2 days; with every 4 of them on 3 days; with every 3 of them on 4 days; with every 2 of them on 5 days. Further every friend was present at 7 dinners and every friend was absent at 7 dinners. How many dinners did I have alone?

### RMO 1994

- A leaf is torn from a paperback novel. The sum of the numbers on the remaining pages is 15000. What are the page numbers on the torn leaf.
- In the triangle ABC, the incircle touches the sides BC, CA and AB respectively at D, E and F. If the radius of the incircle is 4 units and if BD, CE and AF are consecutive integers, find the sides of the triangle ABC.
- Find all 6-digit natural numbers \(a_1a_2a_3a_4a_5a_6 \) formed by using the digits 1, 2, 3, 4, 5, 6,once each such that the number \(a_1a_2a_3…a_k \) is divisible by k, for \(1\le k \le 6 \).
- Solve the system of equations for real x and y : \(5x (1+\frac{1}{x^2+y^2})=12 \) , \(5y(1-\frac{1}{x^2+y^2})=12 \).
- Let A be a set of 16 positive integers with the property that the product of any two distinct numbers of A will not exceed 1994. Show that there are two numbers a and b in A which are not relatively prime.
- Let AC and BD be two chords of a circle with center O such that they intersect at right angles inside the circle at the point M. Suppose K and L are the mid-points of the chord AB and CD respectively. Prove that OKML is a parallelogram.
- Find the number of all rational numbers m/n such that (a) 0 < m/n < 1, (b) m and n are relatively prime, (c) mn = 25!.
- If a, b and c are positive real numbers such that a + b + c = 1, prove that \((1+a)(1+b)(1+c)\ge8(1-a)(1-b)(1-c) \).

### RMO 1995

- In triangle ABC, K and L are points on the side BC (K being closer to B than L) such that BC.KL = BK.CL and AL bisects \(\angle KAC \) . Show that AL is perpendicular to AB.
- Call a positive integer n good if there are n integers, positive or negative, and not necessarily distinct, such that their sum and product are both equal to n (eg. 8 is good, since 8 = 4.2.1.1.1.1.(-1)(-1) = 4 + 2 + 1 + 1 + 1 + 1 + (-1) + (-1) ). Show that integers of the form 4k + 1 and 4l are good.
- Prove that among any 18 consecutive three-digit numbers there is at least one number which is divisible by the sum of it’s digits.
- Show that the quadratic equation \(x^2+7x-14(q^2+1)=0 \) , where q is an integer, has no integer root.
- Show that for any triangle ABC, the following inequality is true: \(a^2+b^2+c^2 > \sqrt{3}max{(|a^2-b^2|,|b^2-c^2|,|c^2-a^2|}) \) where a, b, c are, as usual, the sides of the triangle.
- Let \(A_1A_2A_3….A_{21} \) be a 21-sided reqular polygon inscribed in a circle with center O. How many triangles \(A_iA_jA_k \), contain the point O in their interior?
- Show that for any real number x, \(x^2\sin{x}+x\cos{x}+x^2+\frac{1}{2} > 0 \).

### RMO 1996

- The sides of a triangle are three consecutive integers and its inradius is four units. Determine the circumradius.
- Find all triples (a, b, c) of positive integers such that \((1+\frac{1}{a})(1+\frac{1}{b})(1+\frac{1}{c})=3 \).
- Solve for real number x and y: \(xy^2=15x^2+17xy+15y^2 \), \(x^2y=20x^2+3y^2 \).
- Suppose N is an n-digit positive integer such that (a) all the n-digits are distinct; and (b) the sum of any three consecutive digits is divisible by 5. Prove that n is at most 6. Further, show that starting with any digit one can find a six-digit number with these properties.
- Let ABC be a triangle and ha the altitude through A. Prove that \((b+c)^2 \ge a^2+4h_{a}^{2} \).(As usual a, b, c denote the sides BC, CA, AB respectively.)
- Given any positive integer n show that there are two positive rational numbers a and b,, which are not integers and which are such that a-b, \(a^2-b^2,a^3-b^3,…..,a^n-b^n \) are all integers.
- If A is a fifty-element subset of the set {1, 2, 3, . . . , 100} such that no two numbers from A add up to 100 show that A contains a square.

### RMO 1997

- Let P be an interior point of a triangle ABC and let BP and CP meet AC and AB in E and F respectively. If [BPF] = 4, [BPC] = 8 and [CPE] = 13, find [AFPE]. (Here [·] denotes the area of a triangle or a quadrilateral, as the case may be.)
- For each positive integer n, define \(a_n = 20 + n^2 \), and \(d_n=gcd(a_n,a_{n+1}) \). Find the set of all values that are taken byand show by examples that each of these values are attained.
- Solve for real x: \(\frac{1}{[x]}+\frac{1}{[2x]}=(x)+\frac{1}{3} \),where [x] is the greatest integer less than or equal to x and (x) = x − [x], [e.g. [3.4] = 3 and (3.4) = 0.4].
- In a quadrilateral ABCD, it is given that AB is parallel to CD and the diagonals AC and BD are perpendicular to each other. Show that \((a) AD.BC \ge AB.CD \), \((b) AD+BC \ge AB+CD \).
- Let x, y and z be three distinct real positive numbers. Determine with proof whether or not the three real numbers \(|\frac{x}{y}-\frac{y}{x}|,|\frac{y}{z}-\frac{z}{y}|,|\frac{z}{x}-\frac{x}{z}| \) can be the lengths of the sides of a triangle.
- Find the number of unordered pairs {A,B} (i.e., the pairs {A,B}and{B,A} are considered to be the same) of subsets of an n-element set X which satisfy the conditions: \((a) A \neq B \), \((b) A \cup B =X \). [e.g., if X = {a, b, c, d}, then {{a, b}, {b, c, d}}, {{a}, {b, c, d}}, {, {a, b, c, d}} are some of the admissible pairs.]

### RMO 1998

- Let ABCD be a convex quadrilateral in which \(\angle BAC = 50^0 , \angle CAD = 60^0 , \angle CBD = 30^0 \), and \(\angle BDC = 25^0 \). If E is the point of intersection of AC and BD, find \(\angle AEB \).
- Let n be a positive integer and \(p_1p_2……p_n \)be n prime numbers all larger than 5 such that 6 divides \(p_{1}^{2}+p_{2}^{2}+p_{n}^{2} \). Prove that 6 divides n.
- Prove the following inequality for every natural number n: \(\frac{1}{n+1}(1+\frac{1}{3}+\frac{1}{5}+…..+\frac{1}{2n-1})\ge\frac{1}{n}(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+………+\frac{1}{2n}) \).
- Let ABC be a triangle with AB = BC and \(\angle BAC = 30^o \). Let \(A^* \) be the reflection of A in the line BC; \(B^* \)be the reflection of B in the line CA; \(C^* \) be the reflection of C in the line AB. Show that \(A^*,B^*,C^* \) form the vertices of an equilateral triangle.
- Find the minimum possible least common multiple (lcm) of twenty (not necessarily distinct) natural numbers whose sum is 801.
- Given the 7-element set A = {a, b, c, d, e, f, g}, find a collection T of 3-element subsets of A such that each pair of elements from A occurs exactly in one of the subsets of T.

### RMO 1999

- Prove that the inradius of a right-angled triangle with integer sides is an integer.
- Find the number of positive integers which divide \(10^{999} \) but not \(10^{998} \).
- Let ABCD be a square and M,N points on sides AB,BC, respectably, such that \(\angle MDN = \). If R is the midpoint of MN show that RP = RQ where P,Q are the points of intersection of AC with the lines MD, ND.
- If p, q, r are the roots of the cubic equation \(x^3-3px^2+3q^2x-r^3=0 \), show that p=q=r.
- If a, b, c are the sides of a triangle prove the following inequality: \(\frac{a}{c+a-b}+\frac{b}{a+b-c}+\frac{c}{b+c-a}\ge3 \).
- Find all solutions in integers m, n of the equation \((m-n)^2=\frac{4mn}{m+n-1} \).
- Find the number of quadratic polynomials \(ax^2+bx+c \), which satisfy the following conditions: (a) a, b, c are distinct; (b) a, b, c belongs to {1, 2, 3, . . . 1999} and (c) x + 1 divides \(ax^2+bx+c \).

### RMO 2000

- Let AC be a line segment in the plane and B a point between A and C. Construct isosceles triangles PAB and QBC on one side of the segment AC such that \(\angle APB = \angle BQ= 120^0 \) and an isosceles triangle RAC on the other side of AC such that \(\angle ARC= 120^0 \). Show that PQR is an equilateral triangle.
- Solve the equation \(y^3=x^3+8x^2-6x+8 \),for positive integers x and y.
- Suppose \((x_1, x_2,….., x_n) \) is a sequence of positive real numbers such that \( x_1 \ge x_2 \ge x_3 \ge … \ge x_n… \) and for all n \(\frac{x_1}{1}+\frac{x_4}{2}+\frac{x_9}{3}+….+\frac{x_{n^2}}{n}\le 1 \) . Show that for all k the following inequality is satisfied: \(\frac{x_1}{1}+\frac{x_2}{2}+\frac{x_3}{3}+….+\frac{x_k}{k}\le3 \).
- All the 7-digit numbers containing each of the digits 1, 2, 3, 4, 5, 6, 7 exactly once, and not divisible by 5, are arranged in increasing order. Find the 2000-th number in this list.
- The internal bisector of angle A in a triangle ABC with AC > AB, meets the circumcircle \( \Gamma \) of the triangle in D. Join D to the centre O of the circle \( \Gamma \) and suppose DO meets AC in E, possibly when extended. Given that BE is perpendicular to AD, show that AO is parallel to BD.
- (i) Consider two positive integers a and b which are such that \(a^a b^b \) is divisible by 2000. What is the least possible value of the product ab?

(ii) Consider two positive integers a and b which are such that \(a^a b^b \) is divisible by 2000. What is the least possible value of the product ab? - Find all real values of a for which the equation \(x^4-2ax^2+x+a^2-a=0 \) has all its root real.

### RMO 2001

- Let BE and CF be the altitudes of an acute triangle ABC, with E on AC and F on AB. Let O be the point of intersection of BE and CF. Take any line KL through O with K on AB and L on AC. Suppose M and N are located on BE and CF respectively, such that KM is perpendicular to BE and LN is perpendicular to CF.Prove that FM is parallel to EN.
- Find all primes p and q such that \(p^2+7pq+q^2 \) is the square of an integer.
- Find the number of positive integers x which satisfy the condition \([\frac{x}{99}]=[\frac{x}{101}] \). (Here [z] denotes, for any real z, the largest integer not exceeding z ; e. g. \([\frac{7}{4}] \).
- Consider an n × n array of numbers. Suppose each row consists of the n numbers 1, 2, . . . , n in some order and \(a_{ij}=a_{ji} \) for i = 1, 2, . . . , n and j = 1, 2, . . . , n. If n is odd, prove that the numbers \(a_{11},a_{22},a_{33},….,a_{nn} \) are 1, 2, . . . , n in some order.
- In a triangle ABC , D is a point on BC such that AD is the internal bisector of \(\angle A \). Suppose \(\angle B = 2 \angle C \) and CD = AB. Prove that \(\angle A = 72^o \) .
- If x, y, z are the sides of a triangle, then prove that \( |x^2(y-z)+y^2(z-x)+z^2(x-y)| \le xyz \).
- Prove that the product of the first 1000 positive even integers differs from the product of the first 1000 odd integers by a multiple of 2001.

### RMO 2002

- In an acute triangle ABC, points D;E; F are located on the sides BC;CA;AB respectively such that \(\frac{CD}{CE}=\frac{CA}{CB},\frac{AE}{AF}=\frac{AB}{AC},\frac{BF}{BD}=\frac{BC}{BA} \) Prove that AD;BE;CF are the altitudes of ABC.
- Solve the following equation for real x: \((x^2+x-2)^3+(2x^2-x-1)^3=27(x^2-1)^3 \).
- Let a; b; c be positive integers such that a divides \(b^2 \), b divides \(c^2 \) and c divides \(a^2 \). Prove that abc divides \((a+b+c) \).
- Suppose the integers 1; 2; 3;…; 10 are split into two disjoint collections \(a_1a_2a_3a_4a_5 \) and \(b_1b_2b_3b_4b_5 \) such that \(a_1 < a_2 < a_3 < a_4, b_2 > b_3 > b_4 > b_5 \)

(i) Show that the larger number in any pair { \(a_i\), \(b_j \)}, \(1 \le j \le 5 \) is at least 6.

(ii) Show that \(|a_1-b_1|+|a_2-b_2|+|a_3-b_3|+|a_4-b_4|+|a_5-b_5|=25 \) for every such partition. - The circumference of a circle is divided into eight arcs by a convex quadrilateral ABCD, with four arcs lying inside the quadrilateral and the remaining four lying outside it. The lengths of the arcs lying inside the quadrilateral are denoted by p, q, r, s in counter-clockwise direction starting from some arc. Suppose p + r = q + s. Prove that ABCD is a cyclic quadrilateral.
- For any natural number n > 1, prove the inequality: \({\frac{1}{2}} \le {\frac{1}{1+n^2} + \frac{2}{2+n^2} + \frac{3}{3+n^2} +…..+ \frac{n}{n+n^2}} \le {\frac{1}{2} + \frac{1}{2n}} \)
- Find all integers a; b; c; d satisfying the following relations : (i) \(1 \le a \le b \le c \le d \), (ii) ab + cd = a + b + c + d + 3.

### RMO 2003

- Let ABC be a triangle in which AB = AC and \(\angle CAB = 90^o \). Suppose M and N are points on the hypotenuse BC such that \(BM^2 +CN^2= MN^2 \). Prove that \(\angle MAN = 45^o \).
- If n is an integer greater than 7, prove that \( {n} \choose {7} – \lfloor \frac{n}{7} \rfloor \) is divisible by 7. [ Here \(\frac{n}{7} \) denotes the number of ways of choosing 7 objects from among n objects; also for any real number x, [x] denotes the greatest integer not exceeding x.]
- Let a, b, c be three positive real numbers such that a + b + c = 1. Prove that among the three numbers a − ab, b − bc, c − ca there is one which is at most 1/4 and there is one which is at least 2/9.
- Find the number of ordered triples (x, y, z) of nonnegative integers satisfying the conditions: (i) \(x \le y \le z \), (ii) \(x+y+z \le 100 \).
- Suppose P is an interior point of a triangle ABC such that the ratios d(A,BC) \(\frac{d(A,BC)}{d(P,BC)} , \frac{d(B,CA)}{d(P,CA)},\frac{d(C,AB)}{d(P,AB)} \) are all equal. Find the common value of these ratios. [ Here d(X, Y Z) denotes the perpendicular distance from a point X to the line Y Z.].
- Find all real numbers a for which the equation \(x^2+(a-1)x+1=3|x| \) has exactly three distinct real solutions in x.
- Consider the set X = {1, 2, 3, · · · , 9, 10}. Find two disjoint nonempty subsets A and B of X such that. (a) \(A \cup B = X \); (b) prod(A) is divisible by prod(B), where for any finite set of numbers C, prod(C) denotes the product of all numbers in C; (c)the quotient prod(A)/ prod(B) is as small as possible.

### RMO 2004

- Consider in the plane a circle \(\Gamma \) with center O and a line l not intersecting circle \(\Gamma \). Prove that there is a point Q on the perpendicular drawn from O to the line l, such that for any point P on the line l, PQ represents the length of the tangent from P to the circle \(\Gamma \).
- Positive integers are written on all the faces of a cube, one on each. At each corner (vertex)of the cube, the product of the numbers on the faces that meet at the corner is written. The sum of the numbers written at all the corners is 2004. If T denotes the sum of the numbers on all the faces, find all the possible values of T.
- Let \(\alpha \) and \(\beta \) be the roots of the quadratic equation \(x^2+mx-1 \).where m is an odd integer. Let \(\gamma_m = \alpha^n + \beta^n \),for \(n \ge 0 \). Prove that for \(n\ge0 \), (a) is an integer and (b) \(gcd(\gamma_n, \gamma_{n+1})=1 \).
- Prove that the number of triples (A,B,C) where A,B,C are subsets of {1, 2, · · · , n} such that \(A \cap B \cap C = \phi \), \(A \cap B \neq \phi \), \(B \cap C = \phi \) is \(7^n+2.6^n+5^n \).
- Let ABCD be a quadrilateral ; X and Y be the midpoints of AC and BD respectively ; and the lines through X and Y respectively parallel to BD,AC meet in O. Let P, Q,R, S be the midpoints of AB,BC,CD,DA respectively. Prove that (a) quadrilaterals APOS and APXS have the same area ; (b) the areas of the quadrilaterals APOS,BQOP,CROQ,DSOR are all equal .
- Let \((p_1p_2p_3…..p_n….) \) be a sequence of primes defined by \(p_1=2 \) and for \(n \ge 1 \), \(p_{n+1} \) is the largest prime factor of \(p_1p_2p_3…..p_n+1 \) (Thus \(p_2=3,p_3=7 \)). Prove that \(p_n \neq 5 \) for any n.
- Let x and y be positive real numbers such that \(y^3+y \le x-x^3 \).Prove that (a) y < x < 1; and (b). \(x^2+y^2\le 1 \).

### RMO 2005

- Let ABCD be a convex quadrilateral; P, Q,R, S be the midpoints of AB,BC,CD,DA respectively such that triangles AQR and CSP are equilateral. Prove that ABCD is a rhombus. Determine its angles.
- If x, y are integers and 17 divides both the expressions \(x^2-2xy-y^2-5x+7y \) and \(x^2-3xy+2y^2+x-y \),then prove that 17 divides xy − 12x + 15y.
- If a, b, c are three real numbers such that \(|a-b| \ge c,|b-c| \ge a,|c-a| \ge b \), then prove that one of a, b, c is the sum of the other two.
- Find the number of all 5-digit numbers (in base 10) each of which contains the block 15 and is divisible by 15. (For example, 31545, 34155 are two such numbers.)
- In triangle ABC, let D be the midpoint of BC. If \(\angle ADB = 45^0 \) and \(\angle ACD = 30^0 \), determine \(\angle BAD \).
- Determine all triples (a, b, c) of positive integers such that \(a \le b \le c \) and a + b + c + ab + bc + ca = abc + 1.
- Let a, b, c be three positive real numbers such that a + b + c = 1.Let \(\gamma \)=min{ \(a^3+a^2bc,b^3+ab^2c,c^3+abc^2 \)}.Prove that the roots of the equation \(x^2+x+4 \gamma \) are real.

### RMO 2006

- Let ABC be an acute-angled triangle and let D, E, F be the feet of perpendiculars from A,B,C respectively to BC,CA,AB. Let the perpendiculars from F to CB, CA, AD, BE meet them in P, Q,M,N respectively. Prove that P, Q,M,N are collinear.
- Find the least possible value of a + b, where a, b are positive integers such that 11 divides a + 13b and 13 divides a + 11b.
- If a, b, c are three positive real numbers, prove that \(\frac{a^2+1}{b+c}+\frac{b^2+1}{c+a}+\frac{c^2+1}{a+b} \ge 3 \).
- A 6×6 square is dissected into 9 rectangles by lines parallel to its sides such that all these rectangles have only integer sides. Prove that there are always two congruent rectangles.
- Let ABCD be a quadrilateral in which AB is parallel to CD and perpendicular to AD; AB = 3CD; and the area of the quadrilateral is 4. If a circle can be drawn touching all the sides of the quadrilateral, find its radius.
- Prove that there are infinitely many positive integers n such that n(n+ 1) can be expressed as a sum of two positive squares in at least two different ways. (Here \(a^2+b^2 \) and \(b^2+a^2 \) are considered as the same representation.)
- Let X be the set of all positive integers greater than or equal to 8 and let \(f:X \mapsto X \) be a function such that f(x + y) = f(xy) for all \(x\ge4 \), \(y\ge4 \). If f(8) = 9, determine f(9).

### RMO 2007

- Let ABC be an acute-angled triangle; AD be the bisector of \(angle BAC \) with D on BC; and BE be the altitude from B on AC. Show that \(\angle CED > 45^0 \).
- Let a,b,c be three natural numbers such that \( a\le b \le c \) and \( \gcd(c-a,c-b)=1 \). Suppose that there exits an integer d such that a+d,b+d,c+d form the sides of a right-angled triangle. Prove that there exist integers \(l,m \) such that \(c+d=l^2+m^2 \).
- Find all pairs (a,b) of real numbers such that whenever \(\alpha \)is a root \(x^2+ax+b=0 \), \(\alpha^2-1 \) is also a root of the equation.
- How many 6-digits numbers are there such that: (a) the digits of all the numbers are from the set { \(1,2,3,…. \)}; (b) any digits that appears in the number appears twice? (Example: 225252 is an admissible number, while 222133 is not.)
- A trapezium ABCD, in which AB is parallel to CD, is inscribed in a circle with centre O. Suppose the diagonal AC and BD of the trapezium intersect at M and OM=2. (a) If \(\angle AMB \) is,determine with proof, the difference between the length between the parallel sides. (b) If \(\angle AMD \) is,find the difference between the parallel sides.
- Prove that: (a) \(5 < 5^{\frac{1}{2}}+5^{\frac{1}{3}}+5^{\frac{1}{4}} \); (b) \(8 < 8^{\frac{1}{2}}+8^{\frac{1}{3}}+8^{\frac{1}{4}} \); (c) \(n < n^{\frac{1}{2}}+n^{\frac{1}{3}}+n^{\frac{1}{4}} \) for all integers greater than or equal to 9.

### RMO 2012

- Let ABC be a triangle and D be a point on segment BC such that DC = 2BD. Let E be the midpoint of AC. Let AD and BE intersect in P. Determine the ratios BP/PE and AP/PD.
- Let a, b, c be positive integers such that a divides \(b^3 \) b divides \(c^3 \) and c divides \(a^3 \). Prove that abc divides \((a+b+c)^{13} \).
- Let a and b be positive real numbers such that a+b=1. Prove that \(a^a b^b + a^b b^a\le1 \).
- Let X={1, 2, 3, … , 10}. Find the number of pairs {A, B} such that \(A \in X , B \in X , A\not B \) and \(A \cap B = {2 , 3 , 5, 7 } \).

## RMO 2016 Delhi Region

- Given are two circles \(\omega_1,\omega_2\) which intersect at points\(X,Y\). Let \(P\) be an arbitrary point on \(\omega_1\). Suppose that the lines \(PX,PY\) meet \(\omega_2\) again at points \(A,B\) respectively. Prove that the circumcircles of all triangles \(PAB\) have the same radius.
- Consider a sequence \((a_k)_{k \ge 1}\) of natural numbers defined as follows: \(a_1=a\) and \(a_2=b\) with \(a,b>1\) and \(\gcd(a,b)=1\) and for all \(k>0\), \(a_{k+2}=a_{k+1}+a_k\). Prove that for all natural numbers \(n\) and \(k\), \(\gcd(a_n,a_{n+k}) <\frac{a_k}{2}\).
- Two circles \(C_1\) and \(C_2\) intersect each other at points \(A\) and \(B\). Their external common tangent (closer to \(B\)) touches \(C_1\) at \(P\) and \(C_2\) at \(Q\). Let \(C\) be the reflection of \(B\) in line \(PQ\). Prove that \(\angle CAP=\angle BAQ\).
- Let \(a,b,c\) be positive real numbers such that \(a+b+c=3\). Determine, with certainty, the largest possible value of the expression $$ \frac{a}{a^3+b^2+c}+\frac{b}{b^3+c^2+a}+\frac{c}{c^3+a^2+b}$$
- a.) A 7-tuple \((a_1,a_2,a_3,a_4,b_1,b_2,b_3)\) of pairwise distinct positive integers with no common factor is called a shy tuple if $$ a_1^2+a_2^2+a_3^2+a_4^2=b_1^2+b_2^2+b_3^2$$and for all \(1 \le i<j \le 4\) and \(1 \le k \le 3\), \(a_i^2+a_j^2 \not= b_k^2\). Prove that there exists infinitely many shy tuples.

b.) Show that 2016 can be written as a sum of squares of four distinct natural numbers. - A deck of 52 cards is given. There are four suites each having cards numbered \(1,2,\dots, 13\). The audience chooses some five cards with distinct numbers written on them. The assistant of the magician comes by, looks at the five cards and turns exactly one of them face down and arranges all five cards in some order. Then the magician enters and with an agreement made beforehand with the assistant, he has to determine the face down card (both suite and number). Explain how the trick can be completed.

## RMO 2016 Mumbai Region

- Let \(ABC\) be a right-angled triangle with \(\angle B=90^{\circ}\). Let \(I\) be the incenter of \(ABC\). Draw a line perpendicular to \(AI\) at \(I\). Let it intersect the line \(CB\) at \(D\). Prove that \(CI\) is perpendicular to \(AD\) and prove that \(ID=\sqrt{b(b-a)}\) where \(BC=a\) and \(CA=b\). 1 Let \(ABC\) be a right-angled triangle with \(\angle B=90^{\circ}\). Let \(I\) be the incenter of \(ABC\). Draw a line perpendicular to \(AI\) at \(I\). Let it intersect the line \(CB\) at \(D\). Prove that \(CI\) is perpendicular to \(AD\) and prove that \(ID=\sqrt{b(b-a)}\) where \(BC=a\) and \(CA=b\).
- Let \(a,b,c\) be positive real numbers such that$$\frac{a}{1+a}+\frac{b}{1+b}+\frac{c}{1+c}=1.$$Prove that \(abc \le \frac{1}{8}\).
- For any natural number \(n\), expressed in base \(10\), let \(S(n)\) denote the sum of all digits of \(n\). Find all natural numbers \(n\) such that \(n=2S(n)^2\).
- Find the number of all 6-digits numbers having exactly three odd and three even digits.
- Let \(ABC\) be a triangle with centroid \(G\). Let the circumcircle of triangle \(AGB\) intersect the line \(BC\) in \(X\) different from \(B\); and the circucircle of triangle \(AGC\) intersect the line \(BC\) in \(Y\) different from \(C\). Prove that \(G\) is the centroid of triangle \(AXY\).
- Let \((a_1,a_2,\dots)\) be a strictly increasing sequence of positive integers in arithmetic progression. Prove that there is an infinite sub-sequence of the given sequence whose terms are in a geometric progression.

## RMO 2016 Maharashtra Region

- Find distinct positive integers \(n_1<n_2<\dots<n_7\) with the least possible sum, such that their product \(n_1 \times n_2 \times \dots \times n_7\) is divisible by \(2016\). 1 Find distinct positive integers \(n_1<n_2<\dots<n_7\) with the least possible sum, such that their product \(n_1 \times n_2 \times \dots \times n_7\) is divisible by \(2016\).
- At an international event there are \(100\) countries participating, each with its own flag. There are \(10\) distinct flagpoles at the stadium, labelled 1,#2,…,#10 in a row. In how many ways can all the \(100\) flags be hoisted on these \(10\) flagpoles, such that for each \(i\) from \(1\) to \(10\), the flagpole #i has at least \(i\) flags? (Note that the vertical order of the flagpoles on each flag is important)
- Find all integers \(k\) such that all roots of the following polynomial are also integers:$$f(x)=x^3-(k-3)x^2-11x+(4k-8).$$
- Let \(\triangle ABC\) be scalene, with \(BC\) as the largest side. Let \(D\) be the foot of the perpendicular from \(A\) on side \(BC\). Let points \(K,L\) be chosen on the lines \(AB\) and \(AC\) respectively, such that \(D\) is the midpoint of segment \(KL\). Prove that the points \(B,K,C,L\) are concyclic if and only if \(\angle BAC=90^{\circ}\).
- Let \(x,y,z\) be non-negative real numbers such that \(xyz=1\). Prove that$$(x^3+2y)(y^3+2z)(z^3+2x) \ge 27.$$
- \(ABC\) is an equilateral triangle with side length \(11\) units. Consider the points \(P_1,P_2, \dots, P_10\) dividing segment \(BC\) into \(11\) parts of unit length. Similarly, define \(Q_1, Q_2, \dots, Q_10\) for the side \(CA\) and \(R_1,R_2,\dots, R_10\) for the side \(AB\). Find the number of triples \((i,j,k)\) with \(i,j,k \in \{1,2,\dots,10\}\) such that the centroids of triangles \(ABC\) and \(P_iQ_jR_k\) coincide.

## RMO 2016 Bengal Region

- Let \(ABC\) be a triangle and \(D\) be the mid-point of \(BC\). Suppose the angle bisector of \(\angle ADC\) is tangent to the circumcircle of triangle \(ABD\) at \(D\). Prove that \(\angle A=90^{\circ}\). 1 Let \(ABC\) be a triangle and \(D\) be the mid-point of \(BC\). Suppose the angle bisector of \(\angle ADC\) is tangent to the circumcircle of triangle \(ABD\) at \(D\). Prove that \(\angle A=90^{\circ}\).
- Let \(a,b,c\) be three distinct positive real numbers such that \(abc=1\). Prove that $$\frac{a^3}{(a-b)(a-c)}+\frac{b^3}{(b-c)(b-a)}+\frac{c^3}{(c-a)(c-b)} \ge 3$$
- Let $a,b,c,d,e,d,e,f$ be positive integers such that \(\frac a b < \frac c d < \frac e f\). Suppose \(af-be=-1\). Show that \(d \geq b+f\).
- There are \(100\) countries participating in an olympiad. Suppose \(n\) is a positive integers such that each of the \(100\) countries is willing to communicate in exactly \(n\) languages. If each set of \(20\) countries can communicate in exactly one common language, and no language is common to all \(100\) countries, what is the minimum possible value of \(n\)?
- Let \(ABC\) be a right-angled triangle with \(\angle B=90^{\circ}\). Let \(I\) be the incentre if \(ABC\). Extend \(AI\) and \(CI\); let them intersect \(BC\) in \(D\) and \(AB\) in \(E\) respectively. Draw a line perpendicular to \(AI\) at \(I\) to meet \(AC\) in \(J\), draw a line perpendicular to \(CI\) at \(I\) to meet \(AC\) at \(K\). Suppose \(DJ=EK\). Prove that \(BA=BC\).
- (a). Given any natural number \(N\), prove that there exists a strictly increasing sequence of \(N\) positive integers in harmonic progression.

(b). Prove that there cannot exist a strictly increasing infinite sequence of positive integers which is in harmonic progression.

## RMO 2016 Telengana Region

- Let \(ABC\) be a right angled triangle with \(\angle B=90^{\circ}\). Let \(I\) be the incentre of triangle \(ABC\). Suppose \(AI\) is extended to meet \(BC\) at \(F\) . The perpendicular on \(AI\) at \(I\) is extended to meet \(AC\) at \(E\) . Prove that \(IE = IF\). 1 Let \(ABC\) be a right angled triangle with \(\angle B=90^{\circ}\). Let \(I\) be the incentre of triangle \(ABC\). Suppose \(AI\) is extended to meet \(BC\) at \(F\) . The perpendicular on \(AI\) at \(I\) is extended to meet \(AC\) at \(E\) . Prove that \(IE = IF\).
- Let \(a,b,c\) be positive real numbers such that$$\frac{a}{1+a}+\frac{b}{1+b}+\frac{c}{1+c}=1.$$Prove that \(abc \le \frac{1}{8}\).
- For any natural number \(n\), expressed in base \(10\), let \(S(n)\) denote the sum of all digits of \(n\). Find all positive integers \(n\) such that \(n^3 = 8S(n)^3+6S(n)n+1\).
- Find all \(6\) digit natural numbers, which consist of only the digits \(1,2,\) and \(3\), in which \(3\) occurs exactly twice and the number is divisible by \(9\).
- Let \(ABC\) be a right angled triangle with \(\angle B=90^{\circ}\). Let \(AD\) be the bisector of angle \(A\) with \(D\) on \(BC\) . Let the circumcircle of triangle \(ACD\) intersect \(AB\) again at \(E\); and let the circumcircle of triangle \(ABD\) intersect \(AC\) again at \(F\) . Let \(K\) be the reflection of \(E\) in the line \(BC\) . Prove that \(FK = BC\).
- Show that the infinite arithmetic progression \(\{1,4,7,10 \ldots\}\) has infinitely many 3 -term sub sequences in harmonic progression such that for any two such triples \(\{a_1, a_2 , a_3 \}\) and \(\{b_1, b_2 ,b_3\}\) in harmonic progression , one has$$\frac{a_1} {b_1} \ne \frac {a_2}{b_2}$$