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Geometry Difficulty 6.1 National Olympiad Prove it United States

Problem:

Circles ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c} have centers A,B,CA, B, C, respectively and are pairwise externally tangent at points D,E,FD, E, F (with DBC,ECA,FABD \in BC, E \in CA, F \in AB). Lines BEBE and CFCF meet at TT. Given that ωa\omega_{a} has radius 341341, there exists a line \ell tangent to all three circles, and there exists a circle of radius 4949 tangent to all three circles, compute the distance from TT to \ell.

Solutions — 3

Solution 1

Solution:

We will use the following notation: let ω\omega be the circle of radius 4949 tangent to each of ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c}. Let ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c} have radii ra,rb,rcr_{a}, r_{b}, r_{c} respectively. Let γ\gamma be the incircle of ABCABC, with center II and radius rr. Note that DEFDEF is the intouch triangle of ABCABC and γ\gamma is orthogonal to ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c} (i.e. ID,IE,IFID, IE, IF are the common internal tangents). Since AD,BE,CFAD, BE, CF are concurrent at TT, we have K=ABDEK = AB \cap DE satisfies (A,B;F,K)=1(A, B; F, K) = -1, so KK is the external center of homothety of ωa\omega_{a} and ωb\omega_{b}. In particular, KK lies on \ell. Similarly, BCEFBC \cap EF also lies on \ell, so \ell is the polar of TT to γ\gamma. Hence ITIT \perp \ell so if LL is the foot from II to \ell, we have ITIL=r2IT \cdot IL = r^{2}.

An inversion about γ\gamma preserves ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c} and sends \ell to the circle with diameter ITIT. Since inversion preserves tangency, the circle with diameter ITIT must be ω\omega. Therefore IT=98IT = 98 by the condition of the problem statement. Letting a,b,ca, b, c be the radii of ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c} respectively and invoking Heron's formula as well as A=rsA = r s for triangle ABCABC, we see that γ\gamma has radius

r=rarbrcra+rb+rc. r = \sqrt{\frac{r_{a} r_{b} r_{c}}{r_{a} + r_{b} + r_{c}}}.

We will compute this quantity using Descartes' theorem. Note that there are two circles tangent to ωa\omega_{a}, ωb,ωc\omega_{b}, \omega_{c}, one with radius IT/2IT/2 and one with radius \infty. By Descartes' circle theorem, we have (where ka:=1/ak_{a} := 1/a is the curvature)

ka+kb+kc+2kakb+kbkc+kakc=1IT/2 k_{a} + k_{b} + k_{c} + 2 \sqrt{k_{a} k_{b} + k_{b} k_{c} + k_{a} k_{c}} = \frac{1}{IT/2}

and

ka+kb+kc2kakb+kbkc+kcka=0 k_{a} + k_{b} + k_{c} - 2 \sqrt{k_{a} k_{b} + k_{b} k_{c} + k_{c} k_{a}} = 0

which implies

ra+rb+rcrarbrc=kakb+kbkc+kcka=12IT. \sqrt{\frac{r_{a} + r_{b} + r_{c}}{r_{a} r_{b} r_{c}}} = \sqrt{k_{a} k_{b} + k_{b} k_{c} + k_{c} k_{a}} = \frac{1}{2 IT}.

Therefore r=2ITr = 2 IT, which means IL=r2IT=4ITIL = \frac{r^{2}}{IT} = 4 IT and TL=3IT=294TL = 3 IT = 294.

Figure 1

Solution 2

Solution:

Using the same notation as the previous solution, note that the point TT can be expressed with un-normalized barycentric coordinates

(1ra:1rb:1rc) \left(\frac{1}{r_{a}} : \frac{1}{r_{b}} : \frac{1}{r_{c}}\right)

with respect to ABCABC because TT is the Gergonne point of triangle ABCABC. The distance from TT to \ell can be expressed as a weighted average of the distances from each of the points A,B,CA, B, C, which is

1/ra1/ra+1/rb+1/rcra+1/rb1/ra+1/rb+1/rcrb+1/rc1/ra+1/rb+1/rcrc=31/ra+1/rb+1/rc. \frac{1 / r_{a}}{1 / r_{a} + 1 / r_{b} + 1 / r_{c}} \cdot r_{a} + \frac{1 / r_{b}}{1 / r_{a} + 1 / r_{b} + 1 / r_{c}} \cdot r_{b} + \frac{1 / r_{c}}{1 / r_{a} + 1 / r_{b} + 1 / r_{c}} \cdot r_{c} = \frac{3}{1 / r_{a} + 1 / r_{b} + 1 / r_{c}}.

Note that there are two circles tangent to ωa,ωb,ωc\omega_{a}, \omega_{b}, \omega_{c}, one with radius 4949 and one with radius \infty. By Descartes' circle theorem, we have (where ka:=1/rak_{a} := 1 / r_{a} is the curvature)

ka+kb+kc+2kakb+kbkc+kakc=149 k_{a} + k_{b} + k_{c} + 2 \sqrt{k_{a} k_{b} + k_{b} k_{c} + k_{a} k_{c}} = \frac{1}{49}

and

ka+kb+kc2kakb+kbkc+kcka=0 k_{a} + k_{b} + k_{c} - 2 \sqrt{k_{a} k_{b} + k_{b} k_{c} + k_{c} k_{a}} = 0

so ka+kb+kc=1/98k_{a} + k_{b} + k_{c} = 1 / 98. The distance from TT to \ell is then 398=2943 \cdot 98 = 294.

Solution 3

Solution:

As in the first solution, we deduce that ITIT is a diameter of ω\omega, with TT being the point on ω\omega closest to \ell. By Steiner's porism, we can hold ω\omega and \ell fixed while making ωc\omega_{c} into a line parallel to \ell, resulting in the following figure:

Figure 2

Let RR be the common radius of ωa\omega_{a} and ωb\omega_{b} and rr be the radius of ω\omega. Notice that ABDEABDE is a rectangle with center TT, so R=AE=2IT=4rR = AE = 2 IT = 4r. The distance from TT to \ell is ILIT=2R2r=6r=294IL - IT = 2R - 2r = 6r = 294.

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