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A bicentric quadrilateral, also called a cyclic-inscriptable quadrilateral, is a four-sided bicentric polygon. The inradius , circumradius , and offset are connected by the equation

 (1)

(Davis; Durége 1861; Casey 1888, pp. 109-110; Johnson 1929; Dörie 1965; Coolidge 1971, p. 46; Salazar 2006). Finding this relation is sometimes known as Fuss's problem.

 (2) (3)

(Beyer 1987), where is the semiperimeter, and

 (4)

The area of a bicentric quadrilateral is

 (5) (6)

where and are the lengths of the diagonals (Ivanoff 1960; Beyer 1987, p. 124).

Bicentric Polygon, Bicentric Triangle, Cyclic Quadrilateral, Poncelet's Porism

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## References

Beyer, W. H. (Ed.). CRC Standard Mathematical Tables, 28th ed. Boca Raton, FL: CRC Press, p. 124, 1987.Bogomolny, A. "Easy Construction of Bicentric Quadrilateral." http://www.cut-the-knot.org/Curriculum/Geometry/BicentricQuadri.shtml.Bogomolny, A. "Easy Construction of Bicentric Quadrilateral II." http://www.cut-the-knot.org/Curriculum/Geometry/BicentricQuadri2.shtml.Casey, J. A Sequel to the First Six Books of the Elements of Euclid, Containing an Easy Introduction to Modern Geometry with Numerous Examples, 5th ed., rev. enl. Dublin: Hodges, Figgis, & Co., 1888.Coolidge, J. L. A Treatise on the Geometry of the Circle and Sphere. New York: Chelsea, 1971.Davis, M. A. Educ. Times 32.Dörrie, H. "Fuss' Problem of the Chord-Tangent Quadrilateral." §39 in 100 Great Problems of Elementary Mathematics: Their History and Solutions. New York: Dover, pp. 188-193, 1965.Durége, H. Theorie der elliptischen Functionen: Versuch einer elementaren Darstellung. Leipzig, Germany: Teubner, p. 185, 1861.Ivanoff, V. F. "Solution to Problem E1376: Bretschneider's Formula." Amer. Math. Monthly 67, 291-292, 1960.Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 91-96, 1929.Salazar, J. C. "Fuss's Theorem." Math. Gaz. 90, 306-308, 2006.