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  2. Happy ending problem - Wikipedia

    en.wikipedia.org/wiki/Happy_ending_problem

    In mathematics, the "happy ending problem" (so named by Paul Erdős because it led to the marriage of George Szekeres and Esther Klein [1]) is the following statement: Theorem — any set of five points in the plane in general position [ 2 ] has a subset of four points that form the vertices of a convex quadrilateral .

  3. Sangaku - Wikipedia

    en.wikipedia.org/wiki/Sangaku

    A sangaku dedicated to Konnoh Hachimangu (Shibuya, Tokyo) in 1859.Sangaku or san gaku (Japanese: 算額, lit. 'calculation tablet') are Japanese geometrical problems or theorems on wooden tablets which were placed as offerings at Shinto shrines or Buddhist temples during the Edo period by members of all social classes.

  4. 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]

  5. Langley's Adventitious Angles - Wikipedia

    en.wikipedia.org/wiki/Langley's_Adventitious_Angles

    adventitious quadrangles problem. A quadrilateral such as BCEF is called an adventitious quadrangle when the angles between its diagonals and sides are all rational angles, angles that give rational numbers when measured in degrees or other units for which the whole circle is a rational number. Numerous adventitious quadrangles beyond the one ...

  6. 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).

  7. Japanese theorem for cyclic polygons - Wikipedia

    en.wikipedia.org/wiki/Japanese_theorem_for...

    The quadrilateral case follows from a simple extension of the Japanese theorem for cyclic quadrilaterals, which shows that a rectangle is formed by the two pairs of incenters corresponding to the two possible triangulations of the quadrilateral. The steps of this theorem require nothing beyond basic constructive Euclidean geometry.

  8. Japanese theorem for cyclic quadrilaterals - Wikipedia

    en.wikipedia.org/wiki/Japanese_theorem_for...

    The special case of the theorem for quadrilaterals states that the two pairs of opposite incircles of the theorem above have equal sums of radii. To prove the quadrilateral case, simply construct the parallelogram tangent to the corners of the constructed rectangle, with sides parallel to the diagonals of the quadrilateral.

  9. Ptolemy's inequality - Wikipedia

    en.wikipedia.org/wiki/Ptolemy's_inequality

    Ptolemy's inequality is often stated for a special case, in which the four points are the vertices of a convex quadrilateral, given in cyclic order. [2] [3] However, the theorem applies more generally to any four points; it is not required that the quadrilateral they form be convex, simple, or even planar.

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