Maths Olympiad Prep

Library / /796 of 860

Geometry Difficulty 5.6 AIME, harder Find the answer

Let triangle ABCABC have incircle ω\omega, which touches BC,CABC, CA, and ABAB at D,ED, E, and FF, respectively. Then, let ω1\omega_{1} and ω2\omega_{2} be circles tangent to ADAD and internally tangent to ω\omega at EE and FF, respectively. Let PP be the intersection of line EFEF and the line passing through the centers of ω1\omega_{1} and ω2\omega_{2}. If ω1\omega_{1} and ω2\omega_{2} have radii 5 and 6, respectively, compute PEPFPE \cdot PF.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the centers of ω1\omega_{1} and ω2\omega_{2} be O1O_{1} and O2O_{2}. Let DEDE intersect ω1\omega_{1} again at QQ, and let DFDF intersect ω2\omega_{2} again at RR. Note that since ω1\omega_{1} and ω2\omega_{2} must be tangent to ADAD at the same point (by equal tangents), so ADAD must be the radical axis of ω1\omega_{1} and ω2\omega_{2}, so RQEFRQEF is cyclic. Thus, we have O1QR=EQRO1QE=180EFDO1EQ=90\angle O_{1}QR=\angle EQR-\angle O_{1}QE=180^{\circ}-\angle EFD-\angle O_{1}EQ=90^{\circ} Thus, we have QRQR is tangent to ω1\omega_{1}, and similarly it must be tangent to ω2\omega_{2} as well. Now, note that by Monge's theorem on ω,ω1\omega, \omega_{1}, and ω2\omega_{2}, we have that PP must be the intersection of the external tangents of ω1\omega_{1} and ω2\omega_{2}. Since RQRQ is an external tangent, we have P,QP, Q, and RR are collinear. Thus, by power of a point, we have PEPF=PRPQPE \cdot PF=PR \cdot PQ. Note that PR=1030PR=10 \sqrt{30} and PQ=1230PQ=12 \sqrt{30}. Thus, we have PEPF=3600PE \cdot PF=3600.

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: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.