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The Greek architect Kostas Vittas published in 2006 a beautiful theorem (1) on the cyclic quadrilateral as follows: Theorem 1 (Kostas Vittas, 2006): If ABCD is a cyclic quadrilateral with P being the intersection of two diagonals AC and BD , then the four Euler lines of the triangles PAB , PBC , PCD and PDA are concurrent.
Dergiades et al. (Thu,) studied this question.
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