KnowraEuler's quadrilateral theoremLinked fromLinked fromThe 2 pages that link to Euler's quadrilateral theorem, each with the reason it gives.All 2Compared with 2Ptolemy's theoremCompared with: It applies to general quadrilaterals, while Ptolemy's equality singles out cyclic ones.Ptolemy's inequalityCompared with: It is another general quadrilateral identity, but tracks diagonal geometry rather than bounding their product.