Martin Josefsson, Calculations concerning the tangent lengths and tangency chords of a tangential quadrilateral,
Forum Geometricorum, 10 (2010) 119--130.

Abstract: We derive formulas for the length of the tangency chords and some other quantities in a tangential quadrilateral as functions of the tangent lengths. Three formulas for the area of a bicentric quadrilateral are also proved.

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