Points M and N are the midpoints of the sides BC and AD, respectively, of a convex quadrilateral ABCD. Is it possible that AB+CD>max(AM+DM,BN+CN)
Solution
Answer: no. Since (∠ABC+∠BCD)+(∠BAD+∠CDA)=360∘, one of these summands is not less than 180∘. Without loss of generality, assume that ∠ABC+∠BCD≥180∘. Denote the reflection of the triangle MCD about M by MBD1. The inequality ∠ABM+∠MBE=∠ABC+∠BCD≥180∘ implies that the point B lies either on the segment AE or inside the triangle AME. In both cases AB+BE<AM+ME, which implies AB+CD<AM+DM.
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Source: MathNet,
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