1327. Even more special quadrilaterals

    If a polygon is inscriptible in a circle and circumscriptible about another circle then is a bicentric polygon. The radii of these circles are called inradius and circumradius, and their centers are called incenter and circumcenter... All the triangles are bicentric... If two circles allow a bicentric polygon between them then allow an infinity, all of them with the same number of sides (Poncelet's porism). 

    What condition must fulfill the inradius, the circumradius, the incenter and the circumcenter so that two circles allow a bicentric polygon between them? Nina Guindilla answered:

    Dear Teacher:
    That condition depends on the number of sides of the polygon. I've searched and I've found two theorems. Euler's theorem in Geometry provides the condition for triangles and Fuss's theorem provides the condition for bicentric quadrilaterals:

    If  r  is the inradius,  R  is the circumradius and  d  is the distance between the incenter and the circumcenter then...

Euler's theorem:     1/r = 1/(R+d) + 1/(R–d)

Fuss's theorem:     1/r² = 1/(R+d)² + 1/(R–d)²