Let $\omega$ be the circumcircle of the acute nonisosceles triangle $\Delta ABC$. Point $P$ lies on the altitude from $A$. Let $E$ and $F$ be the feet of the altitudes from P to $CA$, $BA$ respectively. Circumcircle of triangle $\Delta AEF$ intersects the circle $\omega$ in $G$, different from $A$. Prove that the lines $GP$, $BE$ and $CF$ are concurrent.
Problem
Source: Moldova TST 2017, B3
Tags: geometry
06.03.2017 22:56
07.03.2017 01:29
Let $H=AP\cap BC$, since $PFBH$ and $AEPF$ is cyclic we get $\measuredangle ABC=\measuredangle FPA=\measuredangle FEA$ $\Longrightarrow$ $BCEF$ is cyclic, let $A'$ the antipode of $A$ wrt $\omega$ since $PF\parallel A'B$ and $PE\parallel CA'$ we get the perpendicular bisectors of $BF$ and $CE$ intersect in $PA'$ be that point $O$ $\Longrightarrow$ $O$ is the circumcenter of $\odot (BCEF)$ $\Longrightarrow$ $O,P, A'$ are collinear, it so easy note that $G=\overline {OPA'}\cap \omega$. On the other hand by radical axis in $\odot (AFE)$, $\odot (BCEF)$ and $\omega$ we get $AG$, $FE$, $BC$ are concurrent in $X$, let $Y$ $=$ $BE$ $\cap$ $CF$ $\Longrightarrow$ by Brocard's theorem in $\odot (BCEF)$ we get $OY$ $\perp$ $AX$, but $OG$ $\perp$ $AY$ hence $O$ $,$ $G$ $,$ $Y$ are collinear $\Longrightarrow$ $O$ $,$ $P$ $,$ $G$ $,$ $A'$ $,$ $Y$ are collinear hence $GP$, $BE$ and $CF$ are concurrent in $Y$.
03.04.2017 21:05
This is a very common miquel configuration. $\angle AEP + \angle AFP = 90^{\circ}+90^{\circ} = 180^{\circ}$ impies $AEPF$ cyclic. Similarly, we get $PDCE$ and $PDBF$ cyclic where $D$ is the foot of the perpendicular from $A$ to $BC$. From this we get $BCEF$ cyclic and $G$ its miquel point. Let $BE \cap CF = X$. Also, let the center of $BCEF$ be $O$. From the miquel configuration, we have $G, X, O$ colinear. So, we reduce the problem into showing $X, P, O$ colinear. $\angle FEP = \angle FEB - \angle PEX = \angle FCB - 90^{\circ} + \angle BEC = \angle FCB - \angle BCO = \angle FCO$. So, $EP$ meets $CO$ on the circle. Let this point be $Y$. Analogously, $FP$ meets $BO$ at $Z$ which is also on the circle. Now we apply pascal's theorem to hexagon $BZFCYE$. We get our conclusion.
20.08.2020 00:42
Cool problem. Similar solution as above. Firstly we have that $$\angle PEA =\angle PFA = 90^\circ$$from where we obtain that $PEFA$ is cyclic. Let $H$ be feet of altitude from $A$. Now from $$\angle PFB = \angle PHB = 90^\circ$$and $$\angle PEC = \angle PHC = 90^\circ$$we obtain that $PFBH$ and $PEHC$ are cyclic too. Now we are doing some angle chase. $$\angle FBC = 180^\circ - \angle FPH =\angle FPA = \angle FEA = 180^\circ - \angle FEC$$With this we get that $BCFE$ is cyclic. Now let $T = BE \cap CF$. Let $O$ be circumcenter of $BCEF$. $G$ is Miquel point of the quadrilateral and the rest is the same as above so there is no point in writing it all over again
02.10.2020 06:55
See my solution on my Youtube channel here. It is very similar to FabrizioFelen's solution, but with a slight difference in the middle (I think his way is actually simpler). https://www.youtube.com/watch?v=Rl6S_sU7rK4
02.10.2020 11:58
You can work this up using Hong Kong 2001 Test 1 Q1. $\definecolor{A}{RGB}{0,90,255}\color{A}\fbox{Solution}$ As I proved there, if you denote by $O$ the circumcenter of quadrilateral $BFEC$ then points $G,P,O$ lie on line perpendicular to $AG$. Now take $X$ as the intersection of lines $BE,CF$. By radical axis theorem and Brocard's theorem $OX$ is the line perpendicular to $AG$. There's only one line perpendicular to $AG$ passing through point $O$, thus $X\in GP.\blacksquare$ #1802
03.10.2022 15:39
$BEFC$ is cyclic.So from radical center $AG,EF,BC$ concur at $S$. If $BE,CF$ concur at $T$ then $GT,GP$ are perpedicular to $SA$ so $G,T,P$ collinear so we are done
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