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Junior Balkan Mathematical Olympiad Shortlist

Geometry Difficulty 6.3 National Olympiad Prove it JBMO

Problem:
Let ABCABC be a right-angled triangle with A^=90∘\hat{A}=90^{\circ}. Let KK be the midpoint of BCBC, and let AKLMAKLM be a parallelogram with centre CC. Let TT be the intersection of the line ACAC and the perpendicular bisector of BMBM. Let ω1\omega_{1} be the circle with centre CC and radius CACA and let ω2\omega_{2} be the circle with centre TT and radius TBTB. Prove that one of the points of intersection of ω1\omega_{1} and ω2\omega_{2} is on the line LMLM.

Solution

Solution:
Let M′M' be the symmetric point of MM with respect to TT. Observe that TT is equidistant from BB and MM, therefore MM belongs on ω2\omega_{2} and M′MM'M is a diameter of ω2\omega_{2}. It suffices to prove that M′AM'A is perpendicular to LMLM, or equivalently, to AKAK. To see this, let SS be the point of intersection of M′AM'A with LMLM. We will then have ∠M′SM=90∘\angle M'SM=90^{\circ} which shows that SS belongs on ω2\omega_{2} as M′MM'M is a diameter of ω2\omega_{2}. We also have that SS belongs on ω1\omega_{1} as ALAL is diameter of ω1\omega_{1}.

Since TT and CC are the midpoints of M′MM'M and KMKM respectively, then TCTC is parallel to M′KM'K and so M′KM'K is perpendicular to ABAB. Since KA=KBKA=KB, then KM′KM' is the perpendicular bisector of ABAB. But then the triangles KBM′KBM' and KAM′KAM' are equal, showing that ∠M′AK=∠M′BK=∠M′BM=90∘\angle M'AK=\angle M'BK=\angle M'BM=90^{\circ} as required.

Figure 1

Alternative Solution by Proposers.
Since CA=CLCA=CL, then LL belongs on ω1\omega_{1}. Let SS be the other point of intersection of ω1\omega_{1} with the line LMLM. We need to show that SS belongs on ω2\omega_{2}. Since TB=TMTB=TM (TT is on the perpendicular bisector of BMBM) it is enough to show that TS=TMTS=TM.

Let N,T′N, T' be points on the lines ALAL and LMLM respectively, such that MN⊥LMMN \perp LM and TT′⊥LMTT' \perp LM. It is enough to prove that T′T' is the midpoint of SMSM. Since ALAL is diameter of ω1\omega_{1} we have that AS⊥LSAS \perp LS. Thus, it is enough to show that TT is the midpoint of ANAN. We have
AT=AN2⇔AC−CT=AL−LN2⇔2AC−2CT=AL−LN⇔LN=2CT AT=\frac{AN}{2} \Leftrightarrow AC-CT=\frac{AL-LN}{2} \Leftrightarrow 2AC-2CT=AL-LN \Leftrightarrow LN=2CT
as AL=2ACAL=2AC. So it suffices to prove that LN=2CTLN=2CT.

Let DD be the midpoint of BMBM. Since BK=KC=CMBK=KC=CM, then DD is also the midpoint of KCKC. The triangles LMNLMN and CTDCTD are similar since they are right-angled with ∠TCD=∠CAK=∠MLN\angle TCD=\angle CAK=\angle MLN. (AK=KCAK=KC and AKAK is parallel to LMLM.) So we have
LNCT=LMCD=AKCD=CKCD=2 \frac{LN}{CT}=\frac{LM}{CD}=\frac{AK}{CD}=\frac{CK}{CD}=2
as required.

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