::Euclidean geometry


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Detail from Raphael's The School of Athens featuring a Greek mathematician – perhaps representing Euclid or Archimedes – using a compass to draw a geometric construction.

Euclidean geometry is a mathematical system attributed to the Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions (theorems) from these. Although many of Euclid's results had been stated by earlier mathematicians,<ref>Eves, vol. 1., p. 19</ref> Euclid was the first to show how these propositions could fit into a comprehensive deductive and logical system.<ref>Eves (1963), vol. 1, p. 10</ref> The Elements begins with plane geometry, still taught in secondary school as the first axiomatic system and the first examples of formal proof. It goes on to the solid geometry of three dimensions. Much of the Elements states results of what are now called algebra and number theory, explained in geometrical language.<ref>Eves, p. 19</ref>

For more than two thousand years, the adjective "Euclidean" was unnecessary because no other sort of geometry had been conceived. Euclid's axioms seemed so intuitively obvious (with the possible exception of the parallel postulate) that any theorem proved from them was deemed true in an absolute, often metaphysical, sense. Today, however, many other self-consistent non-Euclidean geometries are known, the first ones having been discovered in the early 19th century. An implication of Albert Einstein's theory of general relativity is that physical space itself is not Euclidean, and Euclidean space is a good approximation for it only where the gravitational field is weak.<ref>Misner, Thorne, and Wheeler (1973), p. 47</ref>

Euclidean geometry is an example of synthetic geometry, in that it proceeds logically from axioms to propositions without the use of coordinates. This is in contrast to analytic geometry, which uses coordinates.

Euclidean geometry sections
Intro  The Elements  Methods of proof  System of measurement and arithmetic  Notation and terminology   Some important or well known results   Applications  As a description of the structure of space  Later work  Treatment of infinity  Logical basis  See also   Notes   References  External links  

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