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Equilateral triangle erected on a side defines another

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April 19, 2013
geometryperpendicular bisector

Problem Statement

Let ABCDABCD be a quadrilateral with AD=BCAD = BC and ∠DAB+∠ABC=120∘\angle DAB + \angle ABC = 120^\circ. An equilateral triangle DPCDPC is erected in the exterior of the quadrilateral. Prove that the triangle APBAPB is also equilateral.
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