Maths Olympiad Prep

Track / Stage 7 / 139 of 300 #1539 of 1964

Problem 1539

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.3 Prove it

Let ABCABC be an acute scalene triangle. Let DD and EE be points on the sides ABAB and ACAC, respectively, such that BD=CEBD=CE. Denote by O1O_1 and O2O_2 the circumcentres of the triangles ABEABE and ACDACD, respectively. Prove that the circumcircles of the triangles ABC,ADEABC, ADE, and AO1O2AO_1O_2 have a common point different from AA.

Proposed by Patrik Bak, Slovakia

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. **Identify the common point M M :**
Let M M be the common point of the circumcircles of ABC \triangle ABC and ADE \triangle ADE . Since M M lies on both circumcircles, it must be the center of the spiral similarity that maps BD BD to CE CE . Given that BD=CE BD = CE , M M must be the midpoint of the arc BAC BAC on the circumcircle of ABC \triangle ABC that does not contain A A .

2. **Angle chasing to show O1OA=O2OM \angle O_1OA = \angle O_2OM :**
Since M M is the midpoint of the arc BAC BAC , we have:
BAM=CAM \angle BAM = \angle CAM
This implies that:
BAM=CAM=12BAC \angle BAM = \angle CAM = \frac{1}{2} \angle BAC
Since O1 O_1 and O2 O_2 are the circumcenters of ABE \triangle ABE and ACD \triangle ACD respectively, we have:
O1AO=O2AO=9012BAC \angle O_1AO = \angle O_2AO = 90^\circ - \frac{1}{2} \angle BAC
Therefore:
O1OA=O2OM \angle O_1OA = \angle O_2OM

3. **Prove that OO1=OO2 OO_1 = OO_2 :**
To show that OO1=OO2 OO_1 = OO_2 , we observe the following:
- BOO1BEC \triangle BOO_1 \sim \triangle BEC
- COO2BED \triangle COO_2 \sim \triangle BED

Using the Law of Sines in ABE \triangle ABE and ACD \triangle ACD , we get:
OO1OO2=DCBO1BECO2 \frac{OO_1}{OO_2} = \frac{|DC| \cdot |BO_1|}{|BE| \cdot |CO_2|}
Since BD=CE BD = CE , it follows that:
DCBE=1 \frac{|DC|}{|BE|} = 1
Therefore:
OO1OO2=1    OO1=OO2 \frac{OO_1}{OO_2} = 1 \implies OO_1 = OO_2

4. **Conclude that AO1O2M AO_1O_2M is cyclic:**
Since OO1=OO2 OO_1 = OO_2 and O1OA=O2OM \angle O_1OA = \angle O_2OM , we have:
AOO1MOO2 \triangle AOO_1 \sim \triangle MOO_2
and
MOO1AOO2 \triangle MOO_1 \sim \triangle AOO_2
This implies that AO1O2M AO_1O_2M is an isosceles trapezoid, which is cyclic.

Therefore, the circumcircles of ABC \triangle ABC , ADE \triangle ADE , and AO1O2 \triangle AO_1O_2 have a common point different from A A .

\blacksquare

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.