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Published online by Cambridge University Press: 03 November 2016
In a previous note it has been pointed out as a corollary to the proof of Ptolemy’s Theorem that, if in a cyclic quadrilateral ABCD, AB.CD = BC.DA, and if M be the mid-point of the diagonal BD (2m),
and
so that A and C are each the image in DB of the inverse of the other w.r.t. M, the constant of inversion being m2.
* See M. Ant. Gob, Sur une série de quadrangles. Congrès de Marseilles, 1891.