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Geometry Difficulty 8.7 Shortlist Prove it IMO

Let ABCABC be a triangle, and let 1\ell_{1} and 2\ell_{2} be two parallel lines. For i=1,2i=1,2, let i\ell_{i} meet the lines BCBC, CACA, and ABAB at XiX_{i}, YiY_{i}, and ZiZ_{i}, respectively. Suppose that the line through XiX_{i} perpendicular to BCBC, the line through YiY_{i} perpendicular to CACA, and finally the line through ZiZ_{i} perpendicular to ABAB, determine a non-degenerate triangle Δi\Delta_{i}.
Show that the circumcircles of Δ1\Delta_{1} and Δ2\Delta_{2} are tangent to each other.

Solutions — 2

Solution 1

Throughout the solutions, (p, q)\text{(p, q)} will denote the directed angle between lines pp and qq, taken modulo 180180^{\circ}.
Let the vertices of Δi\Delta_{i} be Di,Ei,FiD_{i}, E_{i}, F_{i}, such that lines EiFiE_{i}F_{i}, FiDiF_{i}D_{i} and DiEiD_{i}E_{i} are the perpendiculars through X,YX, Y and ZZ, respectively, and denote the circumcircle of Δi\Delta_{i} by ωi\omega_{i}.
In triangles D1Y1Z1D_{1}Y_{1}Z_{1} and D2Y2Z2D_{2}Y_{2}Z_{2} we have Y1Z1Y2Z2Y_{1}Z_{1} \parallel Y_{2}Z_{2} because they are parts of 1\ell_{1} and 2\ell_{2}. Moreover, D1Y1D2Y2D_{1}Y_{1} \parallel D_{2}Y_{2} are perpendicular to ACAC and D1Z1D2Z2D_{1}Z_{1} \parallel D_{2}Z_{2} are perpendicular to ABAB, so the two triangles are homothetic and their homothetic centre is Y1Y2Z1Z2=AY_{1}Y_{2} \cap Z_{1}Z_{2} = A. Hence, line D1D2D_{1}D_{2} passes through AA. Analogously, line E1E2E_{1}E_{2} passes through BB and F1F2F_{1}F_{2} passes through CC.
Figure 1
The corresponding sides of Δ1\Delta_{1} and Δ2\Delta_{2} are parallel, because they are perpendicular to the respective sides of triangle ABCABC. Hence, Δ1\Delta_{1} and Δ2\Delta_{2} are either homothetic, or they can be translated to each other. Using that B,X2,Z2B, X_{2}, Z_{2} and E2E_{2} are concyclic, C,X2,Y2C, X_{2}, Y_{2} and F2F_{2} are concyclic, Z2E2ABZ_{2}E_{2} \perp AB and Y2,F2ACY_{2}, F_{2} \perp AC we can calculate
(E 1 E 2 , F 1 F 2 ) = (E 1 E 2 , X 1 X 2 ) + (X 1 X 2 , F 1 F 2 ) = (BE 2 , BX 2 ) + (CX 2 , CF 2 ) = (Z 2 E 2 , Z 2 X 2 ) + (Y 2 X 2 , Y 2 F 2 ) = (Z 2 E 2 , 2 ) + ( 2 , Y 2 F 2 ) = (Z 2 E 2 , Y 2 F 2 ) = (AB, AC) 0, 1\text{(E 1 E 2 , F 1 F 2 ) = (E 1 E 2 , X 1 X 2 ) + (X 1 X 2 , F 1 F 2 ) = (BE 2 , BX 2 ) + (CX 2 , CF 2 ) = (Z 2 E 2 , Z 2 X 2 ) + (Y 2 X 2 , Y 2 F 2 ) = (Z 2 E 2 , 2 ) + ( 2 , Y 2 F 2 ) = (Z 2 E 2 , Y 2 F 2 ) = (AB, AC) 0, 1}
and conclude that lines E1E2E_{1}E_{2} and F1F2F_{1}F_{2} are not parallel. Hence, Δ1\Delta_{1} and Δ2\Delta_{2} are homothetic; the lines D1D2D_{1}D_{2}, E1E2E_{1}E_{2}, and F1F2F_{1}F_{2} are concurrent at the homothetic centre of the two triangles. Denote this homothetic centre by HH.
For i=1,2i=1,2, using (1), and that A,Yi,ZiA, Y_{i}, Z_{i} and DiD_{i} are concyclic,
(HE i , HF i ) = (E 1 E 2 , F 1 F 2 ) = (AB, AC) = (AZ i , AY i ) = (D i Z i , D i Y i ) = (D i E i , D i F i ),\text{(HE i , HF i ) = (E 1 E 2 , F 1 F 2 ) = (AB, AC) = (AZ i , AY i ) = (D i Z i , D i Y i ) = (D i E i , D i F i ),}
so HH lies on circle ωi\omega_{i}.
The same homothety that maps Δ1\Delta_{1} to Δ2\Delta_{2}, sends ω1\omega_{1} to ω2\omega_{2} as well. Point HH, that is the centre of the homothety, is a common point of the two circles, That finishes proving that ω1\omega_{1} and ω2\omega_{2} are tangent to each other.

Solution 2

As in the first solution, let the vertices of Δi\Delta_{i} be Di,Ei,FiD_{i}, E_{i}, F_{i}, such that EiFiE_{i}F_{i}, FiDiF_{i}D_{i} and DiEiD_{i}E_{i} are the perpendiculars through Xi,YiX_{i}, Y_{i} and ZiZ_{i}, respectively. In the same way we conclude that (A,D1,D2),(B,E1,E2)\left(A, D_{1}, D_{2}\right), \left(B, E_{1}, E_{2}\right) and (C,F1,F2)\left(C, F_{1}, F_{2}\right) are collinear.
The corresponding sides of triangles ABCABC and DiEiFiD_{i}E_{i}F_{i} are perpendicular to each other. Hence, there is a spiral similarity with rotation ±90\pm 90^{\circ} that maps ABCABC to DiEiFiD_{i}E_{i}F_{i}; let MiM_{i} be the centre of that similarity. Hence, (M i A, M i D i ) = (M i B, M i E i ) = (M i C, M i F i ) = 90\text{(M i A, M i D i ) = (M i B, M i E i ) = (M i C, M i F i ) = 90}. The circle with diameter ADiAD_{i} passes through Mi,Yi,ZiM_{i}, Y_{i}, Z_{i}, so Mi,A,Yi,Zi,DiM_{i}, A, Y_{i}, Z_{i}, D_{i} are concyclic; analogously (Mi,B,Xi,Zi,EiM_{i}, B, X_{i}, Z_{i}, E_{i}) and (Mi,C,Xi,Yi,FiM_{i}, C, X_{i}, Y_{i}, F_{i}) are concyclic.
By applying Desargues' theorem to triangles ABCABC and DiEiFiD_{i}E_{i}F_{i} we conclude that the lines ADi,BEiAD_{i}, BE_{i} and BFiBF_{i} are concurrent; let their intersection be HH. Since (A,D1,D2),(B,E1,E2)\left(A, D_{1}, D_{2}\right), \left(B, E_{1}, E_{2}\right) and (C,F1,F2)\left(C, F_{1}, F_{2}\right) are collinear, we obtain the same point HH for i=1i=1 and i=2i=2.
Figure 2
By (CB, CH) = (CX i , CF i ) = (Y i X i , Y i F i ) = (Y i Z i , Y i D i ) = (AZ i , AD i ) = (AB, AH)\text{(CB, CH) = (CX i , CF i ) = (Y i X i , Y i F i ) = (Y i Z i , Y i D i ) = (AZ i , AD i ) = (AB, AH)}, point HH lies on circle ABCABC.
Analogously, from (F i D i , F i H ) = (F i Y i , F i C ) = (X i Y i , X i C ) = (X i Z i , X i B ) = (E i Z i , E i B ) = (E i D i , E i H )\text{(F i D i , F i H ) = (F i Y i , F i C ) = (X i Y i , X i C ) = (X i Z i , X i B ) = (E i Z i , E i B ) = (E i D i , E i H )}, we can see that point HH lies on circle DiEiFiD_{i}E_{i}F_{i} as well. Therefore, circles ABCABC and DiEiFiD_{i}E_{i}F_{i} intersect at point HH.
The spiral similarity moves the circle ABCABC to circle DiEiFiD_{i}E_{i}F_{i}, so the two circles are perpendicular. Hence, both circles D1E1F1D_{1}E_{1}F_{1} and D2E2F2D_{2}E_{2}F_{2} are tangent to the radius of circle ABCABC at HH.

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