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Geometry Difficulty 8.3 Shortlist Prove it United States

Let ABC\triangle ABC be a triangle and let DD be a point in its interior. Construct a circle ω1\omega_1 passing through BB and DD and a circle ω2\omega_2 passing through CC and DD such that the point of intersection of ω1\omega_1 and ω2\omega_2 other than DD lies on line ADAD. Denote by EE and FF the points where ω1\omega_1 and ω2\omega_2 intersect side BCBC, respectively, and by XX and YY the intersections of lines DFDF, ABAB and DEDE, ACAC, respectively. Prove that XYBCXY \parallel BC.

Solution

First Solution: Let circles ω1\omega_1 and ω2\omega_2 meet again at RR (other than DD), and let ω1\omega_1 and ω2\omega_2 intersect again with segments ABAB and ACAC, respectively, at PP and QQ (other than BB and CC). By the Power of a Point Theorem, we have APAB=ARAD=AQACAP \cdot AB = AR \cdot AD = AQ \cdot AC; that is, points A,RA, R, and DD lie on the radical axis of the circles ω1\omega_1 and ω2\omega_2. Therefore, points B,C,QB, C, Q, and PP lie on a circle. Consequently, AQP=ABC\angle AQP = \angle ABC. To prove that XYBCXY \parallel BC, it suffices to show that AXY=ABC\angle AXY = \angle ABC, or AQP=ABC=AXY\angle AQP = \angle ABC = \angle AXY; that is, to prove that P,Q,YP, Q, Y, and XX lie on a circle.

Because BCQPBCQP is cyclic, AQP=ABC\angle AQP = \angle ABC. Because BDRPBDRP is cyclic, PDY=PBE=ABC\angle PDY = \angle PBE = \angle ABC. Hence AQP=PDY\angle AQP = \angle PDY, implying that P,Q,YP, Q, Y and DD lie on a circle. Similarly, we can show that P,Q,DP, Q, D, and XX lie on a circle. We conclude that P,Q,Y,DP, Q, Y, D, and XX all lie on the circumcircle of triangle PQDPQD, completing the proof.

Second Solution: It suffices to show that
AXXB=AYYC.() \frac{AX}{XB} = \frac{AY}{YC}. \qquad (*)
Let line ADAD and segment BCBC meet at GG. Applying Menelaus's Theorem to triangle ABGABG and line XFXF, triangle ACGACG and line YEYE gives
AXBFGDXBFGDA=1 and AYCEGDYCEGDA=1. \frac{AX \cdot BF \cdot GD}{XB \cdot FG \cdot DA} = 1 \text{ and } \frac{AY \cdot CE \cdot GD}{YC \cdot EG \cdot DA} = 1.
Hence
AXBFXBFG=AYCEYCEG. \frac{AX \cdot BF}{XB \cdot FG} = \frac{AY \cdot CE}{YC \cdot EG}.
By the equation (*), it suffices to show that
BFFG=CEEG, \frac{BF}{FG} = \frac{CE}{EG},
or EGBF=CEFGEG \cdot BF = CE \cdot FG. By the Power of a point Theorem, we have EGBG=GDGR=GFGCEG \cdot BG = GD \cdot GR = GF \cdot GC, implying that
EGBF=EG(BG+GF)=EGBG+EGGF=GFGC+EGGF=GF(GC+GE)=CEFG, \begin{aligned} EG \cdot BF &= EG(BG + GF) = EG \cdot BG + EG \cdot GF \\ &= GF \cdot GC + EG \cdot GF = GF(GC + GE) \\ &= CE \cdot FG, \end{aligned}
as desired.

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