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Geometry Difficulty 6.6 National Olympiad Prove it Taiwan

ABC\triangle ABC 中, 設點 DDBCBC 邊上且 ADAD 平分 BAC\angle BAC, 並設 ADAD 的中點為 MM。設以 ACAC 為直徑的圓 ω1\omega_1BMBM 交於點 EE, 以 ABAB 為直徑的圓 ω2\omega_2CMCM 交於點 FF。證明 B,E,F,CB, E, F, C 四點共圓。

Let MM be the midpoint of the internal bisector ADAD of ABC\triangle ABC. Circle ω1\omega_1 with diameter ACAC intersects BMBM at EE and circle ω2\omega_2 with diameter ABAB intersects CMCM at FF. Show that B,E,F,CB, E, F, C belong to the same circle.

Solution

If AB=ACAB = AC, then the statement is obvious. Without loss of generality, we assume that AB<ACAB < AC. Let AHAH be the common chord of the given circles, as shown in the figure. We draw the line which passes through AA and is perpendicular to ADAD and denote by KK and LL the points of intersection of the given lines with ω1\omega_1 and ω2\omega_2 respectively.

We prove that BLBL passes through MM. Let XX be the point of intersection of BLBL and ADAD. Since KBADLCKB \parallel AD \parallel LC we have the following proportions:
AXKB=LALK,DXCL=BDBC,BDBC=KALK. \frac{AX}{KB} = \frac{LA}{LK}, \quad \frac{DX}{CL} = \frac{BD}{BC}, \quad \frac{BD}{BC} = \frac{KA}{LK}.
Thus, AX=KBLALKAX = \frac{KB \cdot LA}{LK}, DX=CLKALKDX = \frac{CL \cdot KA}{LK}. Also, KAB=LAC\angle KAB = \angle LAC and triangles AKBAKB and ALCALC are similar. Thus, KALA=KBLC\frac{KA}{LA} = \frac{KB}{LC}, KALC=KBLAKA \cdot LC = KB \cdot LA. Hence, AX=DXAX = DX and XX coincides with MM. By analogy, we can show that CKCK passes through MM too. We have
DME=LMA=CLE=180DHE. \angle DME = \angle LMA = \angle CLE = 180^\circ - \angle DHE.

This implies that E,M,D,HE, M, D, H belong to the same circle. KBFHKBFH is inscribed in the circle ω1\omega_1 and thus
DMF=KMA=MKB=180BHF=DHF, \angle DMF = \angle KMA = \angle MKB = 180^\circ - \angle BHF = \angle DHF,
which implies that M,H,D,FM, H, D, F lie on the circle.

We have shown that M,H,D,F,EM, H, D, F, E lie on the same circle. In the right angled triangle HADHAD, HMHM is a median. Therefore, MD=MHMD = MH. Thus
MDH=MHD=MED=MFH. \angle MDH = \angle MHD = \angle MED = \angle MFH.
Consider triangles MDEMDE and MBDMBD that have the common angle MM and
180CFE=MFE=MDE=MBD. 180^\circ - \angle CFE = \angle MFE = \angle MDE = \angle MBD.
We have that B,E,F,CB, E, F, C are concyclic, which implies the result.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.