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Geometry Difficulty 8.3 Shortlist Prove it Saudi Arabia

Point MM on side ABAB of quadrilateral ABCDABCD is such that quadrilaterals AMCDAMCD and BMDCBMDC are circumscribed around circles centered at O1O_1 and O2O_2 respectively. Line O1O2O_1O_2 cuts an isosceles triangle with vertex MM from angle CMDCMD. Prove that ABCDABCD is a cyclic quadrilateral.

Solution

If ABCDAB \parallel CD then the incircles of AMCDAMCD and BMDCBMDC have equal radii; now the problem conditions imply that the whole picture is symmetric about the perpendicular from MM to O1O2O_1O_2, and hence ABCDABCD is an isosceles trapezoid (or a rectangle). The conclusion in this case is true.

Figure 1

Now suppose that the lines ABAB and CDCD meet at a point KK; we may assume that AA lies between KK and BB. The points O1O_1 and O2O_2 lie on the bisector of the angle BKCBKC. By the problem condition, this angle bisector forms equal angles with the lines CMCM and DMDM; this yields DMK=KCM\angle DMK = \angle KCM. Since O1O_1 and O2O_2 are the incenter of KMCKMC and an excenter of KDMKDM, respectively, we have
DO2K=12DMK=12KCM=DCO1, \angle DO_2K = \frac{1}{2}\angle DMK = \frac{1}{2}\angle KCM = \angle DCO_1,
so the quadrilateral CDO1O2CDO_1O_2 is cyclic. Next, the same points are an excenter of AKDAKD and the incenter of KBCKBC, respectively, so
KAD=2KO1D=2DCO2=KCB; \angle KAD = 2\angle KO_1D = 2\angle DCO_2 = \angle KCB;
this implies the desired cyclicity of the quadrilateral ABCDABCD. \square

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