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  2. Saccheri quadrilateral - Wikipedia

    en.wikipedia.org/wiki/Saccheri_Quadrilateral

    Saccheri quadrilaterals. A Saccheri quadrilateral is a quadrilateral with two equal sides perpendicular to the base.It is named after Giovanni Gerolamo Saccheri, who used it extensively in his 1733 book Euclides ab omni naevo vindicatus (Euclid freed of every flaw), an attempt to prove the parallel postulate using the method reductio ad absurdum.

  3. Hyperbolic geometry - Wikipedia

    en.wikipedia.org/wiki/Hyperbolic_geometry

    The theorems of Alhacen, Khayyam and al-Tūsī on quadrilaterals, including the Ibn al-Haytham–Lambert quadrilateral and Khayyam–Saccheri quadrilateral, were the first theorems on hyperbolic geometry. Their works on hyperbolic geometry had a considerable influence on its development among later European geometers, including Witelo ...

  4. Ultraparallel theorem - Wikipedia

    en.wikipedia.org/wiki/Ultraparallel_theorem

    Let r and s be two ultraparallel lines.. From any two distinct points A and C on s draw AB and CB' perpendicular to r with B and B' on r.. If it happens that AB = CB', then the desired common perpendicular joins the midpoints of AC and BB' (by the symmetry of the Saccheri quadrilateral ACB'B).

  5. Foundations of geometry - Wikipedia

    en.wikipedia.org/wiki/Foundations_of_geometry

    A Saccheri quadrilateral is a quadrilateral which has two sides of equal length, both perpendicular to a side called the base. The other two angles of a Saccheri quadrilateral are called the summit angles and they have equal measure. The summit angles of a Saccheri quadrilateral are acute if the geometry is hyperbolic, and right angles if the ...

  6. Non-Euclidean geometry - Wikipedia

    en.wikipedia.org/wiki/Non-Euclidean_geometry

    A Saccheri quadrilateral is a quadrilateral with two sides of equal length, both perpendicular to a side called the base. The other two angles of a Saccheri quadrilateral are called the summit angles and they have equal measure. The summit angles of a Saccheri quadrilateral are acute if the geometry is hyperbolic, right angles if the geometry ...

  7. Absolute geometry - Wikipedia

    en.wikipedia.org/wiki/Absolute_geometry

    In Euclid's Elements, the first 28 Propositions and Proposition 31 avoid using the parallel postulate, and therefore are valid in absolute geometry.One can also prove in absolute geometry the exterior angle theorem (an exterior angle of a triangle is larger than either of the remote angles), as well as the Saccheri–Legendre theorem, which states that the sum of the measures of the angles in ...

  8. Giordano Vitale - Wikipedia

    en.wikipedia.org/wiki/Giordano_Vitale

    Giordano is most noted nowadays for a theorem on Saccheri quadrilaterals that he proved in his 1668 book Euclide restituto (named after Borelli's Euclides Restitutus of 1658). In examining Borelli's proof of the parallel postulate , Giordano noted that it depended upon the assumption that a line everywhere equidistant from a straight line is ...

  9. Category:Types of quadrilaterals - Wikipedia

    en.wikipedia.org/wiki/Category:Types_of...

    Saccheri quadrilateral; Silver rectangle; Square; T. Tangential quadrilateral; Tangential trapezoid; Trapezoid; U. Unit square This page was last edited on 3 ...