Maths Olympiad Prep

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, 2017

Geometry Difficulty 6.2 National Olympiad Prove it United States

Problem:
Let LBCL B C be a fixed triangle with LB=LCL B = L C, and let AA be a variable point on arc LBL B of its circumcircle. Let II be the incenter of ABC\triangle A B C and AK\overline{A K} the altitude from AA. The circumcircle of IKL\triangle I K L intersects lines KAK A and BCB C again at UKU \neq K and VKV \neq K. Finally, let TT be the projection of II onto line UVU V. Prove that the line through TT and the midpoint of IK\overline{I K} passes through a fixed point as AA varies.

Solution

Solution:
Let MM be the midpoint of arc BCB C not containing LL and let DD be the point where the incircle of triangle ABCA B C touches BCB C. Also let NN be the projection from II to AKA K. We claim that MM is the desired fixed point.
By Simson's Theorem on triangle KUVK U V and point II we have that points T,D,NT, D, N are collinear and since quadrilateral NKDIN K D I is a rectangle we have that line DND N passes through the midpoint of IKI K. Thus it suffices to show that MM lies on line DND N.
Now, let Ia,Ib,IcI_{a}, I_{b}, I_{c} be the A,B,CA, B, C-excenters of triangle ABCA B C respectively. Then II is the orthocenter of triangle IaIbIcI_{a} I_{b} I_{c} and ABCA B C is the Cevian triangle of II with respect to triangle IaIbIcI_{a} I_{b} I_{c}. It's also well-known that MM is the midpoint of IIaI I_{a}.
Let DD' be the reflection of II over BCB C and let NN' be the reflection of II over AKA K. Clearly KK is the midpoint of DND' N'. If we could prove that Ia,D,K,NI_{a}, D', K, N' were collinear then by taking a homothety centered at II with ratio 12\frac{1}{2} we would have that points M,D,NM, D, N were collinear as desired. Thus it suffices to show that points Ia,D,KI_{a}, D', K are collinear.
Let lines BCB C and IbIcI_{b} I_{c} intersect at RR and let lines AIA I and BCB C intersect at SS. Then it's well-known that ( Ib,Ic;A,RI_{b}, I_{c} ; A, R ) is harmonic and projecting from CC we have that (Ia,I;S,A)\left(I_{a}, I ; S, A\right) is harmonic. But KSKAK S \perp K A which means that KSK S bisects angle IKIa\angle I K I_{a}. But it's clear by the definition of DD' that KSK S bisects angle IKD\angle I K D' which implies that points Ia,K,DI_{a}, K, D' are collinear as desired. This completes the proof.

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