Claudi Alsina and Rogen B. Nelsen, On the diagonals of a cyclic quadrilateral,
Forum Geometricorum, 7 (2007) 147--149.

Abstract: We present visual proofs of two lemmas that reduce the proofs of expressions for the lengths of the diagonals and the area of a cyclic quadrilateral in terms of the lengths of its sides to elementary algebra.

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