Geodesic
From equality of Pythagoras to equality of Ptolemy
Matematičeskoe obrazovanie, Tome 97 (2021) no. 1, pp. 2-4

Voir la notice de l'article provenant de la source Math-Net.Ru

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, Tome 97 (2021) no. 1, pp. 2-4. http://geodesic.mathdoc.fr/item/MO_2021_97_1_a0/