GeometryDifficulty 6.1National OlympiadProve itUnited States
Problem:
Circles ωa,ωb,ωc have centers A,B,C, respectively and are pairwise externally tangent at points D,E,F (with D∈BC,E∈CA,F∈AB). Lines BE and CF meet at T. Given that ωa has radius 341, there exists a line ℓ tangent to all three circles, and there exists a circle of radius 49 tangent to all three circles, compute the distance from T to ℓ.
Solutions — 3
Solution 1
Solution:
We will use the following notation: let ω be the circle of radius 49 tangent to each of ωa,ωb,ωc. Let ωa,ωb,ωc have radii ra,rb,rc respectively. Let γ be the incircle of ABC, with center I and radius r. Note that DEF is the intouch triangle of ABC and γ is orthogonal to ωa,ωb,ωc (i.e. ID,IE,IF are the common internal tangents). Since AD,BE,CF are concurrent at T, we have K=AB∩DE satisfies (A,B;F,K)=−1, so K is the external center of homothety of ωa and ωb. In particular, K lies on ℓ. Similarly, BC∩EF also lies on ℓ, so ℓ is the polar of T to γ. Hence IT⊥ℓ so if L is the foot from I to ℓ, we have IT⋅IL=r2.
An inversion about γ preserves ωa,ωb,ωc and sends ℓ to the circle with diameter IT. Since inversion preserves tangency, the circle with diameter IT must be ω. Therefore IT=98 by the condition of the problem statement. Letting a,b,c be the radii of ωa,ωb,ωc respectively and invoking Heron's formula as well as A=rs for triangle ABC, we see that γ has radius
r=ra+rb+rcrarbrc.
We will compute this quantity using Descartes' theorem. Note that there are two circles tangent to ωa, ωb,ωc, one with radius IT/2 and one with radius ∞. By Descartes' circle theorem, we have (where ka:=1/a is the curvature)
Therefore r=2IT, which means IL=ITr2=4IT and TL=3IT=294.
Solution 2
Solution:
Using the same notation as the previous solution, note that the point T can be expressed with un-normalized barycentric coordinates
(ra1:rb1:rc1)
with respect to ABC because T is the Gergonne point of triangle ABC. The distance from T to ℓ can be expressed as a weighted average of the distances from each of the points A,B,C, which is
Note that there are two circles tangent to ωa,ωb,ωc, one with radius 49 and one with radius ∞. By Descartes' circle theorem, we have (where ka:=1/ra is the curvature)
ka+kb+kc+2kakb+kbkc+kakc=491
and
ka+kb+kc−2kakb+kbkc+kcka=0
so ka+kb+kc=1/98. The distance from T to ℓ is then 3⋅98=294.
Solution 3
Solution:
As in the first solution, we deduce that IT is a diameter of ω, with T being the point on ω closest to ℓ. By Steiner's porism, we can hold ω and ℓ fixed while making ωc into a line parallel to ℓ, resulting in the following figure:
Let R be the common radius of ωa and ωb and r be the radius of ω. Notice that ABDE is a rectangle with center T, so R=AE=2IT=4r. The distance from T to ℓ is IL−IT=2R−2r=6r=294.
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