Maths Olympiad Prep

Library / /65 of 92

Geometry Difficulty 6.8 National olympiad Prove it Iran

Two circle ω1\omega_1 and ω2\omega_2 intersect each other at points PP and QQ. An arbitrary line passes from PP and intersects ω1\omega_1 and ω2\omega_2 respectively at points AA and BB. A line parallel to ABAB cuts ω1\omega_1 at DD and FF, and cuts ω2\omega_2 at CC and EE in a way that EE and FF lie between CC and DD. Let XX be the intersection point of BEBE and ADAD, YY be the intersection of AFAF and BCBC, and RR be the reflection of point PP over CDCD.

i)
Prove that RR lies on XYXY.

ii)
Prove that PRPR is the angle bisector of XPY\angle XPY.

Solution

i) Notice that
QEB=QPB=QDA    QEX=QDX \angle QEB = \angle QPB = \angle QDA \implies \angle QEX = \angle QDX
So quadrilateral XDEQXDEQ is cyclic and similarly quadrilateral YCFQYCFQ is cyclic. Let SS be the second intersection point of these two circles. It's clear that QSX=QEB=QCY\angle QSX = \angle QEB = \angle QCY so X,S,YX, S, Y are collinear. Let RR' be the second intersection point of the circumcircle of triangle DSCDSC with line XYXY. We have
RDC=RSC=YSC=YFC=FAP=FDP=CDP \angle R'DC = \angle R'SC = \angle YSC = \angle YFC = \angle FAP = \angle FDP = \angle CDP
Similarly it's obtained that RCD=DCP\angle R'CD = \angle DCP. So RR' is the reflection of PP onto CDCD, and so RRR \equiv R'. □
Figure 1

ii) It's easy to see that FR=FP=ADFR = FP = AD and RD=PD=AFRD = PD = AF. Thus AFRDAFRD is a parallelogram and therefore RDAFRD \parallel AF and RFADRF \parallel AD. Hence,
RYRX=ADDX=PFDX \frac{RY}{RX} = \frac{AD}{DX} = \frac{PF}{DX}
So the problem becomes equivalent to showing that RYRX=PYPX\frac{RY}{RX} = \frac{PY}{PX}. Thus it suffices to show that PYPX=PFDX\frac{PY}{PX} = \frac{PF}{DX}. Note that ADP=AFP\angle ADP = \angle AFP, so PDX=PFY\angle PDX = \angle PFY is enough to show that
PDDX=FYFP    AFFY=DXAD \frac{PD}{DX} = \frac{FY}{FP} \iff \frac{AF}{FY} = \frac{DX}{AD}
From RDAFRD \parallel AF and RFADRF \parallel AD both sides of the last equation are found to be equal to RXRY\frac{RX}{RY} and hence the claim. \square

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.