Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Prove it Saudi Arabia

Given an acute triangle ABCABC inscribed in (O)(O) with A=60\angle A = 60^\circ and symmedian point LL. The tangents at B,CB, C of (O)(O) intersect CA,ABCA, AB at E,FE, F. Prove that the line OLOL, two circumcircles of triangles AEFAEF and BOCBOC pass through the same point.

Solution

Let DD be the midpoint of the minor arc BCBC of (O)(O) and PP be the intersection point of the tangents at B,CB, C of (O)(O). We will show that DD and PP lie on (AEF)(AEF).

Indeed, we have
AEP=AEB=BACABE=ODBADB=ADO \angle AEP = \angle AEB = \angle BAC - \angle ABE = \angle ODB - \angle ADB = \angle ADO
so (ADP)(ADP) passes through EE. Similarly, (ADP)(ADP) passes through FF. Now, let JJ be the center of (AEF)(AEF), and we will show that JDOLJD \parallel OL.

From here, since DD is the center of circle (BOC)(BOC) then JDPXJD \perp PX and OXPXOX \perp PX, it follows that O,X,LO, X, L lie on the same line parallel to JDJD. Since (ABC)(ABC) intersects (AEF)(AEF) at DD other than AA, then ΔDBEΔDCF\Delta DBE \sim \Delta DCF but DB=DCDB = DC so DE=DFDE = DF. On the other hand, JE=JFJE = JF since JJ is the center of (AEF)(AEF) so JDEFJD \perp EF. According to the familiar result, EFOLEF \perp OL, so JDOLJD \parallel OL which finishes the proof. \square

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