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2. 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]

3. Van Aubel's theorem - Wikipedia

   en.wikipedia.org/wiki/Van_Aubel's_theorem

   Follow the quadrilateral vertices in the same sequential direction and construct each square on the left hand side of each side of the given quadrilateral. The segments joining the centers of the squares constructed externally (or internally) to the quadrilateral over two opposite sides have been referred to as Van Aubel segments.

4. Congruence (geometry) - Wikipedia

   en.wikipedia.org/wiki/Congruence_(geometry)

   In geometry, two figures or objects are congruent if they have the same shape and size, or if one has the same shape and size as the mirror image of the other. [ 1 ] More formally, two sets of points are called congruent if, and only if, one can be transformed into the other by an isometry , i.e., a combination of rigid motions , namely a ...

5. Ptolemy's theorem - Wikipedia

   en.wikipedia.org/wiki/Ptolemy's_theorem

   Ptolemy's theorem is a relation among these lengths in a cyclic quadrilateral. = + In Euclidean geometry, Ptolemy's theorem is a relation between the four sides and two diagonals of a cyclic quadrilateral (a quadrilateral whose vertices lie on a common circle).

6. Quadrilateral - Wikipedia

   en.wikipedia.org/wiki/Quadrilateral

   A quadric quadrilateral is a convex quadrilateral whose four vertices all lie on the perimeter of a square. [7] A diametric quadrilateral is a cyclic quadrilateral having one of its sides as a diameter of the circumcircle. [8] A Hjelmslev quadrilateral is a quadrilateral with two right angles at opposite vertices. [9]

7. Newton–Gauss line - Wikipedia

   en.wikipedia.org/wiki/Newton–Gauss_line

   Labels used in proof concerning complete quadrilateral. It is a well-known theorem that the three midpoints of the diagonals of a complete quadrilateral are collinear. [2] There are several proofs of the result based on areas [2] or wedge products [3] or, as the following proof, on Menelaus's theorem, due to Hillyer and published in 1920. [4]