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  2. Euclidean geometry - Wikipedia

    en.wikipedia.org/wiki/Euclidean_geometry

    Euclidean geometry is a mathematical system attributed to ancient Greek mathematician Euclid, which he described in his textbook on geometry, Elements.Euclid's approach consists in assuming a small set of intuitively appealing axioms (postulates) and deducing many other propositions from these.

  3. Straightedge and compass construction - Wikipedia

    en.wikipedia.org/wiki/Straightedge_and_compass...

    In geometry, straightedge-and-compass construction – also known as ruler-and-compass construction, Euclidean construction, or classical construction – is the construction of lengths, angles, and other geometric figures using only an idealized ruler and a pair of compasses.

  4. Playfair's axiom - Wikipedia

    en.wikipedia.org/wiki/Playfair's_axiom

    The classical equivalence between Playfair's axiom and Euclid's fifth postulate collapses in the absence of triangle congruence. [18] This is shown by constructing a geometry that redefines angles in a way that respects Hilbert's axioms of incidence, order, and congruence, except for the Side-Angle-Side (SAS) congruence.

  5. Euclidean planes in three-dimensional space - Wikipedia

    en.wikipedia.org/wiki/Euclidean_planes_in_three...

    In analytic geometry, the intersection of a line and a plane in three-dimensional space can be the empty set, a point, or a line. It is the entire line if that line is embedded in the plane, and is the empty set if the line is parallel to the plane but outside it.

  6. Tarski's axioms - Wikipedia

    en.wikipedia.org/wiki/Tarski's_axioms

    Hilbert's axioms for plane geometry number 16, and include Transitivity of Congruence and a variant of the Axiom of Pasch. The only notion from intuitive geometry invoked in the remarks to Tarski's axioms is triangle. (Versions B and C of the Axiom of Euclid refer to "circle" and "angle," respectively.) Hilbert's axioms also require "ray ...

  7. Nine-point circle - Wikipedia

    en.wikipedia.org/wiki/Nine-point_circle

    The diagram above shows the nine significant points of the nine-point circle. Points D, E, F are the midpoints of the three sides of the triangle. Points G, H, I are the feet of the altitudes of the triangle.

  8. Euclid's lemma - Wikipedia

    en.wikipedia.org/wiki/Euclid's_lemma

    Euclid's lemma — If a prime p divides the product ab of two integers a and b, then p must divide at least one of those integers a or b. For example, if p = 19 , a = 133 , b = 143 , then ab = 133 × 143 = 19019 , and since this is divisible by 19, the lemma implies that one or both of 133 or 143 must be as well.

  9. Euclid's Elements - Wikipedia

    en.wikipedia.org/wiki/Euclid's_Elements

    Euclid's axiomatic approach and constructive methods were widely influential. Many of Euclid's propositions were constructive, demonstrating the existence of some figure by detailing the steps he used to construct the object using a compass and straightedge. His constructive approach appears even in his geometry's postulates, as the first and ...