A quadrilateral which has an incircle, i.e., one for which a single circle can be constructed which is tangent to all four sides. Opposite sides of such a quadrilateral satisfy
 |
(1)
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where
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(2)
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is the semiperimeter, and the area is
 |
(3)
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where 
is the inradius. Using Bretschneider's
formula together with (1) and (3) then
gives the beautiful formula
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(4)
|
 |  |  |
(5)
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where 
and 
are the diagonal lengths.
A rhombus is a special case of a tangential quadrilateral.
