Fig. 1 Let ABCD be a circumscribed quadrilateral, that is, a quadrilateral which has an incircle. Let this incircle have the centerO and the radius ρ and touch its sides AB, BC, CD, DA at the points X, Y, Z, W, respectively. Then, for some very obvious reasons, OX ⊥ AB, OY ⊥ BC, OZ ⊥ CD, OW ⊥ DA and OX = OY = OZ = OW = ρ. Moreover, AW = AX, BX = BY, CY = CZ, DZ = DW, since the two tangents from...

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