Maths Olympiad Prep

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Geometry Difficulty 8.6 Shortlist Prove it Taiwan

For the quadrilateral ABCDABCD, let ACAC and BDBD intersect at EE, ABAB and CDCD intersect at FF, and ADAD and BCBC intersect at GG. Additionally, let W,X,YW, X, Y, and ZZ be the points of symmetry to EE with respect to AB,BC,CDAB, BC, CD, and DADA respectively. Prove that one of the intersection points of (FWY)\odot(FWY) and (GXZ)\odot(GXZ) lies on the line FGFG.

Solution

Consider the statement after inversion at EE: Given a quadrilateral ABCDABCD, let ACAC and BDBD intersect at EE, let (ABE)\odot(ABE) and (CDE)\odot(CDE) intersect at another point FF, let (BCE)\odot(BCE) and (DAE)\odot(DAE) intersect at another point GG, let W,X,Y,ZW, X, Y, Z be the circumcenters of (ABE),(BCE),(CDE),(DAE)\odot(ABE), \odot(BCE), \odot(CDE), \odot(DAE) respectively, prove that: one of the intersection points of (WFY)\odot(WFY) and (XGZ)\odot(XGZ) lies on (EFG)\odot(EFG).

Let MM be the midpoint of WYWY, let SS be the reflection of EE with respect to MM, and let HH be the other intersection point of EFEF and (WFY)\odot(WFY).

Claim 1. EE is the orthocenter of WHY\triangle WHY.
Proof. Note that both WW and YY lie on the perpendicular bisector of EFEF,
(EY,WY)=EYW=WYF=WHF=(WH,HF) \angle(EY, WY) = \angle EYW = \angle WYF = \angle WHF = \angle(WH, HF)
Since WYHFWY \perp HF, it follows that WHEYWH \perp EY, so EE is the orthocenter of WHY\triangle WHY.

Claim 2. MM is the circumcenter of EFG\triangle EFG.
Proof. Since WXBEWX \perp BE and ZYEDZY \perp ED, we get WXYZWX \parallel YZ; similarly we can obtain XYWZXY \parallel WZ, so WXYZWXYZ is a parallelogram, hence MM is also the midpoint of XZXZ. WYWY is the perpendicular bisector of EFEF, XZXZ is the perpendicular bisector of EGEG, and MM is the intersection point of WYWY and XZXZ, so MM is the circumcenter of EFG\triangle EFG.

Claim 3. SS is an intersection point of (WFY)\odot(WFY) and (EFG)\odot(EFG).
Proof. Since MM is the circumcenter of (EFG)\odot(EFG), SS is the antipode of EE on (EFG)\odot(EFG). Also, since EE is WHY\triangle WHY and MM is the midpoint of WYWY, we know that SS is the antipode of HH on (WHY)\odot(WHY). Hence SS lies on (WFY)\odot(WFY) and (EFG)\odot(EFG). Similarly, we can obtain that the reflection of EE with respect to the midpoint of XZXZ lies on (XGZ)\odot(XGZ), and since the midpoint of XZXZ is exactly MM, SS also lies on (XGZ)\odot(XGZ), which completes the proof.

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