Let triangle ABC have incircle ω, which touches BC,CA, and AB at D,E, and F, respectively. Then, let ω1 and ω2 be circles tangent to AD and internally tangent to ω at E and F, respectively. Let P be the intersection of line EF and the line passing through the centers of ω1 and ω2. If ω1 and ω2 have radii 5 and 6, respectively, compute PE⋅PF.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Let the centers of ω1 and ω2 be O1 and O2. Let DE intersect ω1 again at Q, and let DF intersect ω2 again at R. Note that since ω1 and ω2 must be tangent to AD at the same point (by equal tangents), so AD must be the radical axis of ω1 and ω2, so RQEF is cyclic. Thus, we have ∠O1QR=∠EQR−∠O1QE=180∘−∠EFD−∠O1EQ=90∘ Thus, we have QR is tangent to ω1, and similarly it must be tangent to ω2 as well. Now, note that by Monge's theorem on ω,ω1, and ω2, we have that P must be the intersection of the external tangents of ω1 and ω2. Since RQ is an external tangent, we have P,Q, and R are collinear. Thus, by power of a point, we have PE⋅PF=PR⋅PQ. Note that PR=1030 and PQ=1230. Thus, we have PE⋅PF=3600.
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