Poncelet's Porism for Quadrilaterals

Assume circle

A

contains circle

B

. If it is possible to inscribe a quadrilateral touching

A

with its vertices and

B

with its sides, then it is always possible to find a quadrilateral touching both circles and passing through any point of

A

outside

B

. Such quadrilaterals are called bicentric. For this Demonstration circle

A

(in tan) is fixed and the position of circle

B

(in brown) can be changed by dragging the red center (the radius of

B

is obtained through Fuss's formula). You can change the position of the red vertex of the quadrilateral.