Maths Olympiad Prep

Library / /33 of 43

Geometry Difficulty 6.3 National Olympiad Prove it JBMO

Problem:
Let ABCABC be an acute-angled triangle. A circle ω1(O1,R1)\omega_{1}(O_{1}, R_{1}) passes through points BB and CC and meets the sides ABAB and ACAC at points DD and EE, respectively. Let ω2(O2,R2)\omega_{2}(O_{2}, R_{2}) be the circumcircle of the triangle ADEADE. Prove that O1O2O_{1}O_{2} is equal to the circumradius of the triangle ABCABC.

Figure 1

Solution

Solution:
Recall that, in every triangle, the altitude and the diameter of the circumcircle drawn from the same vertex are isogonal. The proof offers no difficulty, being a simple angle chasing around the circumcircle of the triangle.
Let OO be the circumcenter of the triangle ABCABC. From the above, one has OAE=90B\angle OAE = 90^{\circ} - B. On the other hand DEA=B\angle DEA = B, for BCDEBCDE is cyclic. Thus AODEAO \perp DE, implying that in the triangle ADEADE cevians AOAO and AO2AO_{2} are isogonal. So, since AOAO is a radius of the circumcircle of triangle ABCABC, one obtains that AO2AO_{2} is an altitude in this triangle.

Moreover, since OO1OO_{1} is the perpendicular bisector of the line segment BCBC, one has OO1BCOO_{1} \perp BC, and furthermore AO2OO1AO_{2} \parallel OO_{1}.

Chord DEDE is common to ω1\omega_{1} and ω2\omega_{2}, hence O1O2DEO_{1}O_{2} \perp DE. It follows that AOO1O2AO \parallel O_{1}O_{2}, so AOO1O2AOO_{1}O_{2} is a parallelogram. The conclusion is now obvious.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.