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Geometry Difficulty 5.5 AIME, harder Prove it Taiwan

The three vertices X,Y,ZX, Y, Z of an equilateral triangle XYZXYZ lie respectively on the three sides BC,CABC, CA and ABAB of an acute triangle ABCABC.
Prove: the incenter of triangle ABCABC lies inside triangle XYZXYZ.

Solution

Below we prove a stronger result: the incenter II of triangle ABCABC lies inside the incircle of triangle XYZXYZ. Let d(U,VW)d(U, VW) denote the distance from point UU to line VWVW.
Let OO be the incenter of XYZ\triangle XYZ and let r,rr, r' and R,RR', R'' denote respectively the inradius of ABC\triangle ABC, the inradius of XYZ\triangle XYZ, and the circumradius of XYZ\triangle XYZ. Then R=2rR' = 2r', and what we want to prove is equivalent to proving the inequality OIrOI \le r'. Assume OIO \ne I (if O=IO = I the result is obvious).
Let the incircle of ABC\triangle ABC be tangent to sides BC,AC,ABBC, AC, AB at A1,B1,C1A_1, B_1, C_1 respectively. The lines IA1,IB1,IC1IA_1, IB_1, IC_1 divide the plane into 6 acute angles, each of which contains one of the points A1,B1,C1A_1, B_1, C_1. We may assume that OO lies in the angle determined by lines IA1IA_1 and IC1IC_1 that contains the point C1C_1 (as in Figure 1). Let AA' and CC' be respectively the projections of OO onto lines IA1IA_1 and IC1IC_1.
Since OX=ROX = R', d(O,BC)Rd(O, BC) \le R'. Since OABCOA' \parallel BC, we obtain
d(A,BC)=AI+rR or AIRr. d(A', BC) = A'I + r \le R' \text{ or } A'I \le R' - r.
On the other hand, the incircle of ABC\triangle ABC lies inside ABC\triangle ABC. Hence d(O,AB)rd(O, AB) \ge r'. Similarly we obtain
d(O,AB)=CC1=rICr or ICrr. d(O, AB) = C'C_1 = r - IC' \ge r' \text{ or } IC' \le r - r'.
Since both A\angle A' and C\angle C' are right angles, the quadrilateral IAOCIA'OC' is cyclic (as in Figure 1).
On this circle, arc AOCA'OC' =2AIC= 2\angle A'IC' < 180180^\circ = arc OCIOC'I, so 180180^\circ \ge arc IC>IC' > arc AOA'O. This means IC>AOIC' > A'O. From this we obtain
OIIA+AO<IA+IC(Rr)+(rr)=Rr=r , as desired! OI \le IA' + A'O < IA' + IC' \le (R' - r) + (r - r') = R' - r' = r' \text{ , as desired!}

Figure 1
Fig. 1
Figure 2
Fig. 2

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.