Geodesic
From equality of Pythagoras to equality of Ptolemy
Matematičeskoe obrazovanie, no. 1 (2021), pp. 2-4 Cet article a éte moissonné depuis la source Math-Net.Ru

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For a quadrilateral inscribed in a circle, the connection between Ptolemy's equality and the equality of Pythagoras is noted. A formula is obtained for the ratio of the diagonals of the quadrilateral, the lengthes of the diagonals are determined. 
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V. K. Gavrilov. From equality of Pythagoras to equality of Ptolemy. Matematičeskoe obrazovanie, no. 1 (2021), pp. 2-4. http://geodesic.mathdoc.fr/item/MO_2021_1_a0/

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