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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 RR' be the opposite point of RR for diameter. Hence, it's enough to show that RR', PP, CC are collinear. Now assume RPBC=CR'P \cap BC = C' and RPAB=QR'P \cap AB = Q'. The triangles O1ARO_1AR and CABCAB are similar, hence BCRO1=BARA\frac{BC}{RO_1} = \frac{BA}{RA}. The triangles

RQRR'Q'R and CQBC'Q'B are similar, hence BCRR=BQRQ\frac{BC'}{RR'} = \frac{BQ'}{RQ'}.

Figure 1

Calculating length of BCBC, BCBC' and observe that RR=2RO1RR' = 2 \cdot RO_1, BC=BCBC = BC' if and only if
BARQ=2BQRA.(1) BA \cdot RQ' = 2BQ' \cdot RA. \qquad (1)
Let if RPω2=TRP \cap \omega_2 = T, RPω2=TR'P \cap \omega_2 = T', then the points T,TT, T' be the diameters end point of ω2\omega_2. Furthermore, TTRRABTT' \parallel RR' \perp AB, we get that TTTT' passes through the point MM, which is midpoint of ABAB. Thus diameter with QTQ'T circles on the points P,Q,M,TP, Q', M, T.
For above circles we get RQRM=RPRTRQ' \cdot RM = RP \cdot RT and for ω2\omega_2 we have RPRT=RARBRP \cdot RT = RA \cdot RB. From here RQRM=RARBRQ' \cdot RM = RA \cdot RB. In the last equation if we put RM=RA+AB2RM = RA + \frac{AB}{2} and RB=RQ+QBRB = RQ' + Q'B then RQAB2=RAQBRQ' \cdot \frac{AB}{2} = RA \cdot Q'B. Considering with (1) and then CCC \equiv C'.

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