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Geometry Difficulty 8.3 Shortlist Prove it IMO

Let ABCDABCD be a cyclic quadrilateral such that AC<BD<ADAC < BD < AD and DBA<90\angle DBA < 90^{\circ}. Point EE lies on the line through DD parallel to ABAB such that EE and CC lie on opposite sides of line ADAD, and AC=DEAC = DE. Point FF lies on the line through AA parallel to CDCD such that FF and CC lie on opposite sides of line ADAD, and BD=AFBD = AF.
Prove that the perpendicular bisectors of segments BCBC and EFEF intersect on the circumcircle of ABCDABCD.

Solutions — 3

Solution 1

Let TT be the midpoint of arc\overparenBAC\operatorname{arc} \overparen{BAC} and let lines BABA and CDCD intersect EFEF at KK and LL, respectively. Note that TT lies on the perpendicular bisector of segment BCBC.

Figure 1

Since ABCDABCD is cyclic, BDsinBAD=ACsinADC\frac{BD}{\sin \angle BAD} = \frac{AC}{\sin \angle ADC}. From parallel lines we have DAF=ADC\angle DAF = \angle ADC and BAD=EDA\angle BAD = \angle EDA. Hence,
AFsinDAF=BDsinADC=ACsinBAD=DEsinEDA. AF \cdot \sin \angle DAF = BD \cdot \sin \angle ADC = AC \cdot \sin \angle BAD = DE \cdot \sin \angle EDA.
So FF and EE are equidistant from the line ADAD, meaning that EFEF is parallel to ADAD.

We have that KADEKADE and FADLFADL are parallelograms, hence we get KA=DE=ACKA = DE = AC and DL=AF=BDDL = AF = BD. Also, KE=AD=FLKE = AD = FL so it suffices to prove the perpendicular bisector of KLKL passes through TT.

Triangle AKCAKC is isosceles so BTC=BAC=2BKC\angle BTC = \angle BAC = 2 \angle BKC. Likewise, BTC=2BLC\angle BTC = 2 \angle BLC. Since T,KT, K, and LL all lie on the same side of BCBC and TT lies on the perpendicular bisector of BCBC, TT is the centre of circle BKLCBKLC. The result follows.

Solution 2

Let AFAF and DEDE meet ω\omega at XX and YY, respectively, and let TT be as in Solution 1.

As BD<ADBD < AD, DYABDY \parallel AB and BAY=DBA<90\angle BAY = \angle DBA < 90^{\circ}, we have DY<ABDY < AB and YY lies on the opposite side of line ADAD to CC. Also from BD<ADBD < AD, we have B,CB, C, and DD all lie on the same side of the perpendicular bisector of ABAB which shows AC>ABAC > AB. Combining these, we get DY<AB<AC=DEDY < AB < AC = DE and, as YY and EE both lie on the same side of line ADAD, YY lies in the interior of segment DEDE. Similarly, XX lies in the interior of segment DFDF.

Since ABAB is parallel to DYDY, we have YA=BD=FAYA = BD = FA. Likewise XD=AC=EDXD = AC = ED.

Figure 2

Claim 1. TT is the midpoint of arc\overparenXY\operatorname{arc} \overparen{XY}.

Proof. From AXCDAX \parallel CD and ABDYAB \parallel DY we have
CAX=AXD=AYD=YDB. \angle CAX = \angle AXD = \angle AYD = \angle YDB.
Since TT is the midpoint of arc\overparenBAC\operatorname{arc} \overparen{BAC}, we have BAT=TDC\angle BAT = \angle TDC, so
TAX=CAX+BACBAT=YDB+BDCTDC=YDT. \angle TAX = \angle CAX + \angle BAC - \angle BAT = \angle YDB + \angle BDC - \angle TDC = \angle YDT.
Recall from above we have AB<ACAB < AC and analogously, DC<DBDC < DB, which shows that X,YX, Y and TT all lie on the same side of line ADAD. In particular, TT and AA lie on opposite sides of XYXY so TT lies on the internal angle bisector of XAY\angle XAY. Since AF=AYAF = AY, we have ATFATY\triangle ATF \cong \triangle ATY, giving TF=TYTF = TY.

Likewise, TE=TXTE = TX, so TE=TFTE = TF, meaning that TT lies on the perpendicular bisector of segment EFEF as required.

Solution 3

From AF=DBAF = DB, AC=DEAC = DE and
(AC,AF)=(AC,CD)=(AB,BD)=(DE,DB), \angle(AC, AF) = \angle(AC, CD) = \angle(AB, BD) = \angle(DE, DB),
triangles ACFACF and DEBDEB are congruent, so CF=BECF = BE.

Let P=BECFP = BE \cap CF. Since
(CP,BP)=(CF,BE)=(AF,DB)=(DC,DB), \angle(CP, BP) = \angle(CF, BE) = \angle(AF, DB) = \angle(DC, DB),
we have that PP lies on circle ABCDABCD.

Figure 3

Finally, let TT be the Miquel point of the quadrilateral BCFEBCFE so TT lies on circles EFPEFP and ABCDABCD. Note that TT is the centre of spiral similarity taking segments BEBE to CFCF and since BE=CFBE = CF, this is in fact just a rotation, so TB=TCTB = TC and TE=TFTE = TF; that is, the perpendicular bisectors of BCBC and EFEF meet at TT, on circle ABCDABCD.

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