Albrecht Hess, On a circle containing the incenters of tangential quadrilaterals,
Forum Geometricorum, 14 (2014) 389--396.
Abstract. When we fix one side and draw different tangential quadrilaterals having the same side lengths but different angles
we observe that their incenters lie on a circle. Based on a known formula expressing the incircle radius of a tangential
quadrilateral by its tangent lengths, some older results will be presented in a new light and the equation of the
before mentioned circle will appear. This circle encodes information about tangential and bicentric quadrilaterals that
leads to an apparently new characterization of tangential quadrilaterals. Curiously enough, no trigonometric
formulae are needed.
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