Olympiad Maths Prep

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Geometry Difficulty 5.1 AIME, harder Prove it Belarus

Points MM and NN are the midpoints of the sides BCBC and ADAD, respectively, of a convex quadrilateral ABCDABCD. Is it possible that
AB+CD>max(AM+DM,BN+CN) AB + CD > \max(AM + DM, BN + CN)

Solution

Answer: no.
Since (ABC+BCD)+(BAD+CDA)=360(\angle ABC + \angle BCD) + (\angle BAD + \angle CDA) = 360^\circ, one of these summands is not less than 180180^\circ. Without loss of generality, assume that ABC+BCD180\angle ABC + \angle BCD \ge 180^\circ. Denote the reflection of the triangle MCDMCD about MM by MBD1MBD_1. The inequality
ABM+MBE=ABC+BCD180 \angle ABM + \angle MBE = \angle ABC + \angle BCD \ge 180^\circ
implies that the point BB lies either on the segment AEAE or inside the triangle AMEAME. In both cases AB+BE<AM+MEAB + BE < AM + ME, which implies AB+CD<AM+DMAB + CD < AM + DM.

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