In Euclidean geometry, a tangential quadrilateral (or tangent quadrilateral) or circumscribed quadrilateral is a convex quadrilateral whose sides are all tangent to a single circle within the quadrilateral.
Sides of quadrilateral: a,b,c,d
Angle between the diagonals: φ
Radius of inscribed circle: r
- $S=pr=1/2d_1d_2\ sin\ φ$
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