Let $D$ and $E$ be arbitrary points on the sides $BC$ and $AC$ of triangle $ABC$, respectively. The circumcircle of $\triangle ADC$ meets for the second time the circumcircle of $\triangle BCE$ at point $F$. Line $FE$ meets line $AD$ at point $G$, while line $FD$ meets line $BE$ at point $H$. Prove that lines $CF, AH$ and $BG$ pass through the same point.
Problem
Source: JBMO Shortlist 2023, G7
Tags: JBMO, JBMO Shortlist, geometry
29.06.2024 14:15
Let $X=AD \cap BE$, $K=AH \cap (ACDF)$ and $L=BG \cap (BCEF)$. From the cyclic quadrilaterals we get that: $\angle HKF = \angle AKF = \angle ACF = \angle ECF = \angle EBF = \angle HBF => HKBF$ is cyclic. Hence we get that $\angle XBK = \angle HBK = \angle HFK = \angle DFK = \angle DAK = \angle XAK$, and so $BKXA$ is cyclic. Analogously, $ALXB$, is cyclic and hence $A,L,X,K,B$ are all concyclic. Now from radical axis on $(CDKFA),(CELFB)$ and $(ALXKB)$ we get that $CF,AK,BL$ are concurrent and hence $CF,AH,BG$ are concurrent
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01.07.2024 04:32
Here's a solution with Ceva's theorem We just need to prove that $\frac{\sin \angle ACF}{\sin \angle BCF}\cdot\frac{\sin \angle CBG}{\sin \angle ABG}\cdot\frac{\sin \angle BAH}{\sin \angle CAH}=1$. Notice that $\angle EBF=\angle ACF=\angle ADF$, $\angle BEF=\angle BCF=\angle DAF$, hence $\triangle BEF\sim \triangle DAF$, then $\triangle BDF\sim \triangle EAF$. Then, $$\frac{\sin \angle ACF}{\sin \angle BCF}=\frac{\sin \angle ADF}{\sin \angle DAF}=\frac{AF}{DF}=\frac{AE}{BD}$$$$\frac{\sin \angle CBG}{\sin \angle ABG}=\frac{DG}{AG}\cdot\frac{AB}{BD},\frac{\sin \angle BAH}{\sin \angle CAH}=\frac{BH}{HE}\cdot\frac{AE}{AB}$$Hence, $$\frac{\sin \angle ACF}{\sin \angle BCF}\cdot\frac{\sin \angle CBG}{\sin \angle ABG}\cdot\frac{\sin \angle BAH}{\sin \angle CAH}=\frac{AE}{BD}\cdot\frac{DG}{AG}\cdot\frac{AB}{BD}\cdot\frac{BH}{HE}\cdot\frac{AE}{AB}=\frac{AE^2}{BD^2}\cdot\frac{DG}{AG}\cdot\frac{BH}{HE}=1$$In the last step we used that $$\frac{AE}{BD}\cdot\frac{DG}{AG}=\frac{\sin \angle DFG}{\sin \angle AFG}=\frac{\sin \angle EFH}{\sin \angle BFH}=\frac{BD}{AE}\cdot \frac{HE}{BH}$$Hence we are done. $\square$
01.07.2024 23:28
Also given at North Macedonia Balkan MO 2024 TST and some revenge ELMOs at MOP.
30.07.2024 10:24
As some students found, just one Pappus finishes this problem without using the definition of point F(and my solution was like #3).
04.11.2024 07:22
Indeed, the circles $ACDF$ and $BCEF$ are totally not needed, the point $F$ could be as arbitrary as we wish. Applying Pappus' theorem to the lines $ADG$ and $BEH$ gives that $AE \cap BD = C$, $DH \cap EG = F$ and $AH \cap BG$ are collinear, done. :X Good exercise for Pappus, but I am very glad this one was not selected for JBMO (not that it had much of a chance, as it involved radical axis), otherwise it would have been shameful for the competition.