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Geometry Difficulty 7.2 National Olympiad, round 2 Prove it Russia

Diagonals of a convex quadrilateral ABCD intersect at E. The four points of tangency of the circles (ABE) and (CDE) with their external common tangents lie on a circle ω\omega. Analogously, the four points of tangency of the circles (ADE) and (BCE) with their external common tangents lie on a circle γ\gamma. Prove that the centers of ω\omega and γ\gamma coincide.

(А. Д. Терёшин)

Solution

Let us denote the centers of the circumscribed circles of triangles ABE, BCE, CDE, ADE by OABO_{AB}, OBCO_{BC}, OCDO_{CD}, OADO_{AD} respectively. Let T1,T2T_1, T_2 be the points of tangency of one of the common tangents with the circumscribed circles of triangles ABE and CDE, respectively; denote by O and T the midpoints of segments OABOCDO_{AB}O_{CD} and T1T2T_1T_2, respectively (см. рис. 1). Then, in the right trapezoid OABT1T2OCDO_{AB}T_1T_2O_{CD}, the line OTOT is the midline, hence it is the perpendicular bisector of segment T1T2T_1T_2. Note that the circle ω\omega is symmetric with respect to the line OABOCDO_{AB}O_{CD}, on which the point OO also lies, meaning that OO is the center of ω\omega.

Figure 1
Рис. 1

Figure 2
Рис. 2

Similarly, we find that the midpoint of segment OADOBCO_{AD}O_{BC} is the center of γ\gamma. Therefore, the statement of the problem is equivalent to the fact that OABOBCOCDOADO_{AB}O_{BC}O_{CD}O_{AD} is a parallelogram. To prove this, it suffices to note that OABOBCO_{AB}O_{BC} and OCDOADO_{CD}O_{AD} are the perpendicular bisectors of segments EBEB and EDED, respectively, hence OABOBCOCDOADO_{AB}O_{BC} \parallel O_{CD}O_{AD}; similarly, OABOADOBCOCDO_{AB}O_{AD} \parallel O_{BC}O_{CD}, from which the required conclusion follows.

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