Bretschneider's formula
Suppose we have a quadrilateral with edges of length 
(in that order) and diagonals of length 
. Bretschneider's formula states that the area
![$[ABCD]=\frac{1}{4}*\sqrt{4p^2q^2-(b^2+d^2-a^2-c^2)^2}$](http://latex.artofproblemsolving.com/7/5/9/7596249e635c730943d0f5493ab8e0d9f2399293.png)
.
It can be derived with vector geometry.
Proof
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See Also
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