Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
@FedorPetrov The distance between incenter and circumcenter. Also radius of circumscribed circle is the same for all quadrilaterals in the family because distance between incenter and circumcenter, inradius and radius of circumscribed circle are related by Fuss theorem.