Maths Olympiad Prep

Library / /14 of 16

Geometry Difficulty 7.4 National olympiad, round 2 Prove it Czech-Polish-Slovak Mathematical Match

Given the triangle ABCABC, let kk be the excircle at the side BCBC. Choose any line pp parallel to BCBC intersecting line segments ABAB and ACAC at points DD and EE. Denote by ll the incircle of the triangle ADEADE. The tangents from DD and EE to the circle kk not passing through AA intersect at PP. The tangents from BB and CC to the circle ll not passing through AA intersect at QQ. Prove that the line PQPQ passes through a fixed point independent of the choice of pp.

Solution

Let BCBC touch kk in TkT_k and DEDE touch ll in TlT_l. We shall prove the required fixed point is TkT_k.

First we show the points TkT_k, TlT_l and PP are collinear. Denote by UU and VV the points where EPEP and DPDP touch kk, and MM and NN the points where EPEP and DPDP intersect BCBC. Let T1T_1 and T2T_2 be the tangent points of kk and the rays ABAB and ACAC respectively.

Figure 1
Fig. 1

As BCDEBC \parallel DE, the triangle DEPDEP is similar to NMPNMP and the homothety HH with the centre PP and the quotient q=MN/EDq = MN/ED maps the segment DEDE into NMNM. To prove the collinearity of Tk,Tl,PT_k, T_l, P, it suffices to derive the equality
MTkNTk=ETlDTl(1) \frac{MT_k}{NT_k} = \frac{ET_l}{DT_l} \qquad (1)
if this is true, HH maps TlT_l into TkT_k.

Let a,b,ca, b, c be the lengths of the sides in the triangle DEPDEP as in fig. 1. Set AD=cAD = c, AE=dAE = d. Let us remind the well-known formulae for the length of the segments between the vertex of a triangle and the tangent points of its incircle and excircle: In any triangle XYZXYZ, the distance of XX from the tangent point of the incircle and excircle (lying on XYXY) is (XY+XZYZ)/2(XY + XZ - YZ)/2 and (XY+YZXZ)/2(XY + YZ - XZ)/2 respectively.

The circle kk is the excircle of NMPNMP. Hence
MTkNTk=(MN+NPMP)/2(MN+MPNP)/2=qa+qcqbqa+qbqc=a+cba+bc(2) \frac{MT_k}{NT_k} = \frac{(MN + NP - MP)/2}{(MN + MP - NP)/2} = \frac{qa + qc - qb}{qa + qb - qc} = \frac{a+c-b}{a+b-c} \qquad (2)
The circle ll is the incircle of DEADEA. Hence
ETlDTl=(DE+AEAD)/2(DE+ADAE)/2=a+dca+cd(3) \frac{ET_l}{DT_l} = \frac{(DE + AE - AD)/2}{(DE + AD - AE)/2} = \frac{a+d-c}{a+c-d} \qquad (3)
When we draw two tangent lines from a point to a circle, the distances of the two tangent points from the original point are equal. Repeating this argument several times we get
c+c+PU=e+c+PV=e+DTlAT1AT2=d+ET2d+b+PU c + c + PU = e + c + PV = e + DT_l \quad AT_1 \cdot AT_2 = d + ET_2 \cdot d + b + PU
so e+c=d+be + c = d + b. Then cb=dec - b = d - e and substituting into (2) and (3) gives
MTkNTk=a+(cb)a(cb)=a+(de)a(de)=ETlDTl \frac{MT_k}{NT_k} = \frac{a + (c - b)}{a - (c - b)} = \frac{a + (d - e)}{a - (d - e)} = \frac{ET_l}{DT_l}
which is exactly (1). Hence PP lies on T1TkT_1T_k.

Similarly we show that also TkT_k, TlT_l and QQ are collinear. Denote by UU' and VV' the points where CQCQ and BQBQ touch ll, and MM' and NN' the points where CQC'Q and BQB'Q intersect DEDE. Let T1T_1' and T2T_2' be the tangent points of ll and the rays ADAD and AEAE respectively. Let a,b,ca', b', c' be the lengths of the sides in the triangle BCQBCQ and AB=cAB = c', AC=dAC = d'.

Figure 2
Fig. 2

Repeating the arguments from the first part (here both circles are excircles) we get
MTlNTl=a+cba+bcCTkBTk=a+cda+dc \frac{M'T_l}{N'T_l} = \frac{a' + c' - b'}{a' + b' - c'} \qquad \frac{CT_k}{BT_k} = \frac{a' + c' - d'}{a' + d' - c'}
Comparing the lengths (fig. 2) gives
ccQU=ccQV=cBT1=AT1=AT2=dCT2=dbQU c' - c' - QU' = c' - c' - QV' = c' - BT_{1}' = AT_{1}' = AT_{2}' = d' - CT_{2}' = d' - b' - QU'
so cb=cdc' - b' = c' - d' and
MTlNTl=CTkBTk \frac{M'T_l}{N'T_l} = \frac{CT_k}{BT_k}
Finally, using the homothety of BCQBCQ and NMQN'M'Q we conclude that QQ lies on TlTkT_lT_k.

Therefore the line PQPQ (obviously, PQP \neq Q) is identical to the line TlTkT_lT_k and passes through TkT_k, which is independent of the choice of pp.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.