# IMO Geomety Problems. (IMO 1999/1) Determine all finite sets S of at least three points in the plane which satisfy the following condition:

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1 IMO Geomety Problems (IMO 1999/1) Determine all finite sets S of at least three points in the plane which satisfy the following condition: for any two distinct points A and B in S, the perpendicular bisector of the line segment AB is an axis of symmetry for S. (IMO 1999/5) Two circles G 1 and G 2 are contained inside the circle G, and are tangent to G at the distinct points M and N, respectively. G 1 passes through the center of G 2. The line passing through the two points of intersection of G 1 and G 2 meets G at A and B. The lines MA and MB meet G 1 at C and D, respectively. Prove that CD is tangent to G 2. (IMO 1998/1) In the convex quadrilateral ABCD, the diagonals AC and BD are perpendicular and the opposite sides AB and DC are not parallel. Suppose that the point P, where the perpendicular bisectors of AB and DC meet, is inside ABCD. Prove that ABCD is a cyclic quadrilateral if and only if the triangles ABP and CDP have equal areas. (IMO 1998/5) Let I be the incenter of triangle ABC. Let the incircle of ABC touch the sides BC, CA, and AB at K, L, and M, respectively. The line through B parallel to MK meets the lines LM and LK at R and S, respectively. Prove that angle RIS is acute. (IMO 1997/2) The angle at A is the smallest angle of triangle ABC. The points B and C divide the circumcircle of the triangle into two arcs. Let U be an interior point of the arc between B and C which does not contain A. The perpendicular bisectors of AB and AC meet the line AU at V and W, respectively. The lines BV and CW meet at T. Show that AU = T B + T C. (IMO 1996/2) Let P be a point inside triangle ABC such that AP B ACB = AP C ABC. Let D, E be the incenters of triangles AP B, AP C, respectively. AP, BD, CE meet at a point. Show that (IMO 1996/5) Let ABCDEF be a convex hexagon such that AB is parallel to DE, BC is parallel to EF, and CD is parallel to F A. Let R A, R C, R E denote the circumradii of triangles F AB, BCD, DEF, respectively, and let P denote the perimeter of the hexagon. Prove that R A + R C + R E P 2.

2 (IMO 1995/1) Let A, B, C, D be four distinct points on a line, in that order. The circles with diameters AC and BD intersect at X and Y. The line XY meets BC at Z. Let P be a point on the line XY other than Z. The line CP intersects the circle with diameter AC at C and M, and the line BP intersects the circle with diameter BD at B and N. Prove that the lines AM, DN, XY are concurrent. (IMO 1995/5) Let ABCDEF be a convex hexagon with AB = BC = CD and DE = EF = F A, such that BCD = EF A = π/3. Suppose G and H are points in the interior of the hexagon such that AGB = DHE = 2π/3. Prove that AG + GB + GH + DH + HE CF. (IMO 1994/2) ABC is an isosceles triangle with AB = AC. Suppose that M is the midpoint of BC and O is the point on the line AM such that OB is perpendicular to AB; Q is an arbitrary point on the segment BC different from B and C; E lies on the line AB and F lies on the line AC such that E, Q, F are distinct and collinear. (IMO 1993/2) Let D be a point inside acute triangle ABC such that ADB = ACB + π/2 and AC BD = AD BC. (a) Calculate the ratio (AB CD)/(AC BD). (b) Prove that the tangents at C to the circumcircles of ACD and BCD are perpendicular. (IMO 1993/4) For three points P, Q, R in the plane, we define m(p QR) as the minimum length of the three altitudes of P QR. (If the points are collinear, we set m(p QR) = 0.) Prove that for points A, B, C, X in the plane, m(abc) m(abx) + m(axc) + m(xbc). (IMO 1992/4) In the plane let C be a circle, L a line tangent to the circle C, and M a point on L. Find the locus of all points P with the following property: there exists two points Q, R on L such that M is the midpoint of QR and C is the inscribed circle of triangle P QR. (IMO 1991/1) Given a triangle ABC, let I be the center of its inscribed circle. The internal bisectors of the angles A, B, C meet the opposite sides in A, B, C respectively. Prove that 1 4 < AI BI CI AA BB CC (IMO 1991/5) Let ABC be a triangle and P an interior point of ABC. Show that at least one of the angles P AB, P BC, P CA is less than or equal to 30.

3 (IMO 1990/1) Chords AB and CD of a circle intersect at a point E inside the circle. Let M be an interior point of the segment EB. The tangent line at E to the circle through D, E, and M intersects the lines BC and AC at F and G, respectively. If AM AB = t, find EG EF in terms of t. (IMO 1989/2) In an acute-angled triangle ABC the internal bisector of angle A meets the circumcircle of the triangle again at A 1. Points B 1 and C 1 are defined similarly. Let A 0 be the point of intersection of the line AA 1 with the external bisectors of angles B and C. Points B 0 and C 0 are defined similarly. Prove that: (i) The area of the triangle A 0 B 0 C 0 is twice the area of the hexagon AC 1 BA 1 CB 1. (ii) The area of the triangle A 0 B 0 C 0 is at least four times the area of the triangle ABC. (IMO 1989/4) Let ABCD be a convex quadrilateral such that the sides AB, AD, BC satisfy AB = AD+BC. There exists a point P inside the quadrilateral at a distance h from the line CD such that AP = h + AD and BP = h + BC. Show that h AD BC (IMO 1988/1) Consider two coplanar circles of radii R and r (R > r) with the same center. Let P be a fixed point on the smaller circle and B a variable point on the larger circle. The line BP meets the larger circle again at C. The perpendicular l to BP at P meets the smaller circle again at A. (If l is tangent to the circle at P then A = P.) (i) Find the set of values of BC 2 + CA 2 + AB 2. (ii) Find the locus of the midpoint of BC. (IMO 1988/5) ABC is a triangle right-angled at A, and D is the foot of the altitude from A. The straight line joining the incenters of the triangles ABD, ACD intersects the sides AB, AC at the points K, L respectively. S and T denote the areas of the triangles ABC and AKL respectively. Show that S 2T. (IMO 1987/2) In an acute-angled triangle ABC the interior bisector of the angle A intersects BC at L and intersects the circumcircle of ABC again at N. From point L perpendiculars are drawn to AB and AC, the feet of these perpendiculars being K and M respectively. Prove that the quadrilateral AKN M and the triangle ABC have equal areas.

4 (IMO 1986/2) A triangle A 1 A 2 A 3 and a point P 0 are given in the plane. We define A s = A s 3 for all s 4. We construct a set of points P 1, P 2, P 3,..., such that P k+1 is the image of P k under a rotation with center A k+1 through angle 120 clockwise (for k = 0, 1, 2,... ). Prove that if P 1986 = P 0, then the triangle A 1 A 2 A 3 is equilateral. (IMO 1986/4) Let A, B be adjacent vertices of a regular n-gon (n 5) in the plane having center at O. A triangle XY Z, which is congruent to and initially conincides with OAB, moves in the plane in such a way that Y and Z each trace out the whole boundary of the polygon, X remaining inside the polygon. Find the locus of X. (IMO 1985/5) A circle with center O passes through the vertices A and C of triangle ABC and intersects the segments AB and BC again at distinct points K and N, respectively. The circumscribed circles of the triangles ABC and KBN intersect at exactly two distinct points B and M. Prove that angle OMB is a right angle. (IMO 1984/4) Let ABCD be a convex quadrilateral such that the line CD is a tangent to the circle on AB as diameter. Prove that the line AB is a tangent to the circle on CD as diameter if and only if the lines BC and AD are parallel. (IMO 1983/2) Let A be one of the two distinct points of intersection of two unequal coplanar circles C 1 and C 2 with centers O 1 and O 2, respectively. One of the common tangents to the circles touches C 1 at P 1 and C 2 at P 2, while the other touches C 1 at Q 1 and C 2 at Q 2. Let M 1 be the midpoint of P 1 Q 1,and M 2 be the midpoint of P 2 Q 2. Prove that O 1 AO 2 = M 1 AM 2. (IMO 1982/2) A non-isosceles triangle A 1 A 2 A 3 is given with sides a 1, a 2, a 3 (a i is the side opposite A i ). For all i = 1, 2, 3, M i is the midpoint of side a i, and T i. is the point where the incircle touches side a i. Denote by S i the reflection of T i in the interior bisector of angle A i. Prove that the lines M 1 S 1, M 2 S 2, and M 3 S 3 are concurrent. (IMO 1981/1) P is a point inside a given triangle ABC.D, E, F are the feet of the perpendiculars from P to the lines BC, CA, AB respectively. Find all P for which BC P D + CA P E + AB P F is least. (IMO 1981/5) Three congruent circles have a common point O and lie inside a given triangle. Each circle touches a pair of sides of the triangle. Prove that the incenter and the circumcenter of the triangle and the point O are collinear. (IMO 1979/3) Two circles in a plane intersect. Let A be one of the points of intersection. Starting simultaneously from A two points move with constant speeds, each point travelling along its own circle in the same sense. The two points return to A simultaneously after one revolution. Prove that there is a

5 fixed point P in the plane such that, at any time, the distances from P to the moving points are equal. (IMO 1979/4) Given a plane π, a point P in this plane and a point Q not in π, find all points R in π such that the ratio (QP + P R)/QR is a maximum. (IMO 1978/2) P is a given point inside a given sphere. Three mutually perpendicular rays from P intersect the sphere at points U, V, and W ; Q denotes the vertex diagonally opposite to P in the parallelepiped determined by P U, P V, and P W. Find the locus of Q for all such triads of rays from P (IMO 1978/4) In triangle ABC, AB = AC. A circle is tangent internally to the circumcircle of triangle ABC and also to sides AB, AC at P, Q, respectively. Prove that the midpoint of segment P Q is the center of the incircle of triangle ABC. (IMO 1977/1) Equilateral triangles ABK, BCL, CDM, DAN are constructed inside the square ABCD. Prove that the midpoints of the four segments KL, LM, MN, NK and the midpoints of the eight segments AKBK, BL, CL, CM, DM, DN, AN are the twelve vertices of a regular dodecagon. (IMO 1976/1) In a plane convex quadrilateral of area 32, the sum of the lengths of two opposite sides and one diagonal is 16. Determine all possible lengths of the other diagonal. (IMO 1975/3) On the sides of an arbitrary triangle ABC, triangles ABR, BCP, CAQ are constructed externally with CBP = CAQ = 45, BCP = ACQ = 30, ABR = BAR = 15. Prove that QRP = 90 and QR = RP. (IMO 1974/2) In the triangle ABC, prove that there is a point D on side AB such that CD is the geometric mean of AD and DB if and only if sin A sin B sin 2 C 2. (IMO 1972/2) Prove that if n 4, every quadrilateral that can be inscribed in a circle can be dissected into n quadrilaterals each of which is inscribable in a circle. (IMO 1971/4) All the faces of tetrahedron ABCD are acute-angled triangles. We consider all closed polygonal paths of the form XY ZT X defined as follows: X is a point on edge AB distinct from A and B; similarly, Y, Z, T are interior points of edges BCCD, DA, respectively. Prove: (a) If DAB + BCD CDA + ABC, then among the polygonal paths, there is none of minimal length. (b) If DAB + BCD = CDA + ABC, then there are infinitely many shortest polygonal paths, their common length being 2AC sin(α/2), where α = BAC + CAD + DAB.

6 (IMO 1970/1) Let M be a point on the side AB of ABC. Let r 1, r 2 and r be the radii of the inscribed circles of triangles AMC, BMC and ABC. Let q 1, q 2 and q be the radii of the escribed circles of the same triangles that lie in the angle ACB. Prove that r 1 r2 = r q 1 q 2 q. (IMO 1970/5) In the tetrahedron ABCD, angle BDC is a right angle. Suppose that the foot H of the perpendicular from D to the plane ABC is the intersection of the altitudes of ABC. Prove that (AB + BC + CA) 2 6(AD 2 + BD 2 + CD 2 ). For what tetrahedra does equality hold? (IMO 1969/4) A semicircular arc γ is drawn on AB as diameter. C is a point on γ other than A and B, and D is the foot of the perpendicular from C to AB. We consider three circles, γ 1, γ 2, γ 3, all tangent to the line AB. Of these, γ 1 is inscribed in ABC, while γ 2 and γ 3 are both tangent to CD and to γ, one on each side of CD. Prove that γ 1, γ 2 and γ 3 have a second tangent in common. (IMO 1968/4) Prove that in every tetrahedron there is a vertex such that the three edges meeting there have lengths which are the sides of a triangle. (IMO 1967/1) Let ABCD be a parallelogram with side lengths AB = a, AD = 1, and with BAD = α. If ABD is acute, prove that the four circles of radius 1 with centers A, B, C, D cover the parallelogram if and only if a cos α + 3 sin α. (IMO 1967/4) Let A 0 B 0 C 0 and A 1 B 1 C 1 be any two acute-angled triangles. Consider all triangles ABC that are similar to A 1 B 1 C 1 (so that vertices A 1, B 1, C 1 correspond to vertices A, B, C, respectively) and circumscribed about triangle A 0 B 0 C 0 (where A 0 lies on BC, B 0 on CA, and AC 0 on AB). Of all such possible triangles, determine the one with maximum area, and construct it. (IMO 1966/3) The sum of the distances of the vertices of a regular tetrahedron from the center of its circumscribed sphere is less than the sum of the distances of these vertices from any other point in space. (IMO 1965/3) Given the tetrahedron ABCD whose edges AB and CD have lengths a and b respectively. The distance between the skew lines AB and CD is d, and the angle between them is ω. Tetrahedron ABCD is divided into two solids by plane ε, parallel to lines AB and CD. The ratio of the distances of ε from AB and CD is equal to k. Compute the ratio of the volumes of the two solids obtained. (IMO 1965/5) Consider OAB with acute angle AOB. Through a point M O perpendiculars are drawn to OA and OB, the feet of which are P and Q

7 respectively. The point of intersection of the altitudes of OP Q is H. What is the locus of H if M is permitted to range over (a) the side AB, (b) the interior of OAB? (IMO 1964/3) A circle is inscribed in triangle ABC with sides a, b, c. Tangents to the circle parallel to the sides of the triangle are constructed. Each of these tangents cuts off a triangle from ABC. In each of these triangles, a circle is inscribed. Find the sum of the areas of all four inscribed circles (in terms of a, b, c). (IMO 1964/6) In tetrahedron ABCD, vertex D is connected with D 0 the centroid of ABC. Lines parallel to DD 0 are drawn through A, B and C. These lines intersect the planes BCD, CAD and ABD in points A 1, B 1 and C 1, respectively. Prove that the volume of ABCD is one third the volume of A 1 B 1 C 1 D 0. Is the result true if point D 0 is selected anywhere within ABC? (IMO 1963/2) Point A and segment BC are given. Determine the locus of points in space which are vertices of right angles with one side passing through A, and the other side intersecting the segment BC. (IMO 1962/5) On the circle K there are given three distinct points A, B, C. Construct (using only straightedge and compasses) a fourth point D on K such that a circle can be inscribed in the quadrilateral thus obtained. (IMO 1961/4) Consider triangle P 1 P 2 P 3 and a point P within the triangle. Lines P 1 P, P 2 P, P 3 P intersect the opposite sides in points Q 1, Q 2, Q 3 respectively. Prove that, of the numbers P 1 P P Q 1, P 2P P Q 2, P 3P P Q 3 at least one is 2 and at least one is 2. (IMO 1961/5) Construct triangle ABC if AC = b, AB = c and AMB = ω, where M is the midpoint of segment BC and ω < 90. Prove that a solution exists if and only if b tan ω 2 c < b. (IMO 1960/3) In a given right triangle ABC, the hypotenuse BC, of length a, is divided into n equal parts (n an odd integer). Let α be the acute angle subtending, from A, that segment which contains the midpoint of the hypotenuse. Let h be the length of the altitude to the hypotenuse of the triangle. Prove: tan α = 4nh (n 2 1)a. (IMO 1960/5) Consider the cube ABCDA B C D (with face ABCD directly above face A B C D ).

8 (a) Find the locus of the midpoints of segments XY, where X is any point of AC and Y is any point of B D. (b) Find the locus of points Z which lie on the segments XY of part (a) with ZY = 2XZ. (IMO 1959/5) An arbitrary point M is selected in the interior of the segment AB. The squares AMCD and MBEF are constructed on the same side of AB, with the segments AM and M B as their respective bases. The circles circumscribed about these squares, with centers P and Q, intersect at M and also at another point N. Let N denote the point of intersection of the straight lines AF and BC. (a) Prove that the points N and N coincide. (b) Prove that the straight lines MN pass through a fixed point S independent of the choice of M. (c) Find the locus of the midpoints of the segments P Q as M varies between A and B. (IMO 1959/6) Two planes, P and Q, intersect along the line p. The point A is given in the plane P, and the point C in the plane Q; neither of these points lies on the straight line p. Construct an isosceles trapezoid ABCD (with AB parallel to CD) in which a circle can be inscribed, and with vertices B and D lying in the planes P and Q respectively.

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