Brahmagupta's Formula
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Brahmagupta's Formula is a formula for determining the area of a cyclic quadrilateral given only the four side lengths.
Definition
Given a cyclic quadrilateral with side lengths 
, 
, 
, 
, the area 
can be found as:
where 
is the semiperimeter of the quadrilateral.
Proof
This proof is incomplete. You can help us out by completing it.
Similar formulas
Bretschneider's formula gives a formula for the area of a non-cyclic quadrilateral given only the side lengths; applying Ptolemy's Theorem to Bretschneider's formula reduces it to Brahmagupta's formula.
Brahmagupta's formula reduces to Heron's formula by setting the side length 
.
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