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rectangle ABCD so that AB = AD and AB + BC <CD, prove <ABC >120^o

Source: INAMO Shortlist 2015 G1

May 7, 2019
rectanglegeometryanglegeometric inequality

Problem Statement

Given a cyclic quadrilateral ABCDABCD so that AB=ADAB = AD and AB+BC<CDAB + BC <CD. Prove that the angle ABCABC is more than 120120 degrees.
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