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2. Orthodiagonal quadrilateral - Wikipedia

   en.wikipedia.org/wiki/Orthodiagonal_quadrilateral

   The area K of an orthodiagonal quadrilateral equals one half the product of the lengths of the diagonals p and q: [8] K = p q 2 . {\displaystyle K={\frac {pq}{2}}.} Conversely, any convex quadrilateral where the area can be calculated with this formula must be orthodiagonal. [ 6 ]

3. List of formulas in elementary geometry - Wikipedia

   en.wikipedia.org/wiki/List_of_formulas_in...

   Perimeter#Formulas – Path that surrounds an area; List of second moments of area; List of surface-area-to-volume ratios – Surface area per unit volume; List of surface area formulas – Measure of a two-dimensional surface; List of trigonometric identities; List of volume formulas – Quantity of three-dimensional space

4. Ptolemy's theorem - Wikipedia

   en.wikipedia.org/wiki/Ptolemy's_theorem

   More generally, if the quadrilateral is a rectangle with sides a and b and diagonal d then Ptolemy's theorem reduces to the Pythagorean theorem. In this case the center of the circle coincides with the point of intersection of the diagonals. The product of the diagonals is then d 2, the right hand side of Ptolemy's relation is the sum a 2 + b 2.

5. Quadrilateral - Wikipedia

   en.wikipedia.org/wiki/Quadrilateral

   This reduces to Brahmagupta's formula for the area of a cyclic quadrilateral—when A + C = 180°. Another area formula in terms of the sides and angles, with angle C being between sides b and c, and A being between sides a and d, is = ⁡ + ⁡.

6. Euler's quadrilateral theorem - Wikipedia

   en.wikipedia.org/wiki/Euler's_quadrilateral_theorem

   Euler's quadrilateral theorem or Euler's law on quadrilaterals, named after Leonhard Euler (1707–1783), describes a relation between the sides of a convex quadrilateral and its diagonals. It is a generalisation of the parallelogram law which in turn can be seen as generalisation of the Pythagorean theorem .

7. Bretschneider's formula - Wikipedia

   en.wikipedia.org/wiki/Bretschneider's_formula

   Bretschneider's formula generalizes Brahmagupta's formula for the area of a cyclic quadrilateral, which in turn generalizes Heron's formula for the area of a triangle.. The trigonometric adjustment in Bretschneider's formula for non-cyclicality of the quadrilateral can be rewritten non-trigonometrically in terms of the sides and the diagonals e and f to give [2] [3]