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Mongolian Mathematical Olympiad

Geometry Difficulty 7.0 National Olympiad, round 2 Prove it Mongolia

Given ω1,ω2\omega_1, \omega_2 circles centered with O1,O2O_1, O_2 and external tangent to each other at point PP. A tangent line ll with ω1\omega_1 and ω2\omega_2 meet at points A,BA, B. Also, a line passing through BB with perpendicular to ll and line O,AO, A meet at CC. The line PCPC and segment ABAB met at QQ, line O,QO, Q and segment BCBC meet at DD. Prove that DD is midpoint of BCBC segment.
(proposed by B. Battsengel)

Solution

Let RR be the line ll tangent to ω1\omega_1, in the ω1\omega_1 R′R' be the opposite point of RR for diameter. Hence, it's enough to show that R′R', PP, CC are collinear. Now assume R′P∩BC=C′R'P \cap BC = C' and R′P∩AB=Q′R'P \cap AB = Q'. The triangles O1ARO_1AR and CABCAB are similar, hence BCRO1=BARA\frac{BC}{RO_1} = \frac{BA}{RA}. The triangles

R′Q′RR'Q'R and C′Q′BC'Q'B are similar, hence BC′RR′=BQ′RQ′\frac{BC'}{RR'} = \frac{BQ'}{RQ'}.

Figure 1

Calculating length of BCBC, BC′BC' and observe that RR′=2⋅RO1RR' = 2 \cdot RO_1, BC=BC′BC = BC' if and only if
BA⋅RQ′=2BQ′⋅RA.(1) BA \cdot RQ' = 2BQ' \cdot RA. \qquad (1)
Let if RP∩ω2=TRP \cap \omega_2 = T, R′P∩ω2=T′R'P \cap \omega_2 = T', then the points T,T′T, T' be the diameters end point of ω2\omega_2. Furthermore, TT′∥RR′⊥ABTT' \parallel RR' \perp AB, we get that TT′TT' passes through the point MM, which is midpoint of ABAB. Thus diameter with Q′TQ'T circles on the points P,Q′,M,TP, Q', M, T.
For above circles we get RQ′⋅RM=RP⋅RTRQ' \cdot RM = RP \cdot RT and for ω2\omega_2 we have RP⋅RT=RA⋅RBRP \cdot RT = RA \cdot RB. From here RQ′⋅RM=RA⋅RBRQ' \cdot RM = RA \cdot RB. In the last equation if we put RM=RA+AB2RM = RA + \frac{AB}{2} and RB=RQ′+Q′BRB = RQ' + Q'B then RQ′⋅AB2=RA⋅Q′BRQ' \cdot \frac{AB}{2} = RA \cdot Q'B. Considering with (1) and then C≡C′C \equiv C'.

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