” In Euclidean Geometry, Ptolemy theorem is a relationship between the four sides and the two diagonals of a cyclic quadrilateral who’s vertices lie on a common circle”

“if the quadrilateral is given with its four vertices as follows B,C,D,E then the product of the measure of its diagonals is equal to the sum of the product of the opposite pair sides”

BD*CE=CB*ED+CD*BE

https://www.geogebra.org/m/Qa5s9HS8