geometry problems from India IITB Mathathon, with aops links in the names
2021 R2,R3 collected inside aops here
2019, 2021
2018 IITB Mathathon Round 4 Sample (2013 Rioplatense Olympiad, Level 3, p2)
Let ABCD be a square, and let E and F be points in AB and BC respectively such that BE=BF. In the triangle EBC, let N be the foot of the altitude relative to EC. Let G be the intersection between AD and the extension of the previously mentioned altitude. FG and EC intersect at point P, and the lines NF and DC intersect at point T. Suppose DP meets BC at X. Prove that X is orthocentre of triangle BTD.
2019 IITB Mathathon Round 1 p8 (2016 HMMT Team p10)
Let ABC be a triangle with incenter I whose incircle is tangent to \overline{BC}, \overline{CA}, \overline{AB} at D, E, F. Point P lies on \overline{EF} such that \overline{DP} \perp \overline{EF}. Ray BP meets \overline{AC} at Y and ray CP meets \overline{AB} at Z. Point Q is selected on the circumcircle of \triangle AYZ so that \overline{AQ} \perp \overline{BC}. Prove that P, I, Q are collinear.
2019 IITB Mathathon Round 4 p2 (USA TSTST 2018 p5)
Let ABC be an acute triangle with circumcircle \omega, and let H be the foot of the altitude from A to \overline{BC}. Let P and Q be the points on \omega with PA = PH and QA = QH. The tangent to \omega at P intersects lines AC and AB at E_1 and F_1 respectively; the tangent to \omega at Q intersects lines AC and AB at E_2 and F_2 respectively. Show that the circumcircles of \triangle AE_1F_1 and \triangle AE_2F_2 are congruent, and the line through their centers is parallel to the tangent to \omega at A.
by Ankan Bhattacharya and Evan Chen
In \Delta ABC , AB = 12 , AC = 13 and BC = 14. Let I be the incenter. The circle with diameter AI meets (ABC) again at D. Let external angle bisector of \angle BDC intersect BC at E. Find the length of EB.
A triangle \triangle ABC has its incenter I. The lines AI,BI,CI intersect the circumcircle of \triangle ABC in D,E,F resp. Let M,N,P be the circumcenters of triangles IEF, IDF and IDE respectively. Prove that the lines DM, EN and FP are concurrent.
In \triangle ABC, AC>AB. Let D be the point on segment AC such that CD=AB. Let the internal angle bisector of \angle BAC intersect \odot(ABC) at M. Let the perpendicular bisector of segment AD intersect AM at E, and let F be the midpoint of segment EM. Prove that FD=FB.
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