MathDB
2018-2019 Fall OMO Problem 28

Source:

November 7, 2018

Problem Statement

Let ω\omega be a circle centered at OO with radius R=2018R=2018. For any 0<r<10090 < r < 1009, let γ\gamma be a circle of radius rr centered at a point II satisfying OI=R(R2r)OI =\sqrt{R(R-2r)}. Choose any A,B,CωA,B,C\in \omega with AC,ABAC, AB tangent to γ\gamma at E,FE,F, respectively. Suppose a circle of radius rAr_A is tangent to AB,ACAB,AC, and internally tangent to ω\omega at a point DD with rA=5rr_A=5r. Let line EFEF meet ω\omega at P1,Q1P_1,Q_1. Suppose P2,P3,Q2,Q3P_2,P_3,Q_2,Q_3 lie on ω\omega such that P1P2,P1P3,Q1Q2,Q1Q3P_1P_2,P_1P_3,Q_1Q_2,Q_1Q_3 are tangent to γ\gamma. Let P2P3,Q2Q3P_2P_3,Q_2Q_3 meet at KK, and suppose KIKI meets ADAD at a point XX. Then as rr varies from 00 to 10091009, the maximum possible value of OXOX can be expressed in the form abc\frac{a\sqrt{b}}{c}, where a,b,ca,b,c are positive integers such that bb is not divisible by the square of any prime and gcd(a,c)=1\gcd (a,c)=1. Compute 10a+b+c10a+b+c.
Proposed by Vincent Huang