Roger A. For many years, this elementary treatise on advanced Euclidean geometry has been the standard textbook in this area of classical mathematics; no other book has covered the subject quite as well. It explores the geometry of the triangle and the circle, concentrating on extensions of Euclidean theory, and examining in detail many relatively recent theorems. Because of Euclidean geometry's fundamental status in mathematics, it is impractical to give more than a representative sampling of applications here. Sphere packing applies to a stack of oranges.

As suggested by the etymology of the word, one of the earliest reasons for interest in geometry was surveying , [20] and certain practical results from Euclidean geometry, such as the right-angle property of the triangle, were used long before they were proved formally. Historically, distances were often measured by chains, such as Gunter's chain , and angles using graduated circles and, later, the theodolite.

An application of Euclidean solid geometry is the determination of packing arrangements , such as the problem of finding the most efficient packing of spheres in n dimensions. This problem has applications in error detection and correction. Geometric optics uses Euclidean geometry to analyze the focusing of light by lenses and mirrors. The water tower consists of a cone, a cylinder, and a hemisphere.

Its volume can be calculated using solid geometry. Geometry is used extensively in architecture. Geometry can be used to design origami.

Some classical construction problems of geometry are impossible using compass and straightedge , but can be solved using origami. Design geometry typically consists of shapes bounded by planes, cylinders, cones, tori, etc. A few decades ago, sophisticated draftsmen learned some fairly advanced Euclidean geometry, including things like Pascal's theorem and Brianchon's theorem.

But now they don't have to, because the geometric constructions are all done by CAD programs. Euclid believed that his axioms were self-evident statements about physical reality. Euclid's proofs depend upon assumptions perhaps not obvious in Euclid's fundamental axioms, [23] in particular that certain movements of figures do not change their geometrical properties such as the lengths of sides and interior angles, the so-called Euclidean motions , which include translations, reflections and rotations of figures.

As discussed in more detail below, Einstein 's theory of relativity significantly modifies this view. The ambiguous character of the axioms as originally formulated by Euclid makes it possible for different commentators to disagree about some of their other implications for the structure of space, such as whether or not it is infinite [26] see below and what its topology is. Modern, more rigorous reformulations of the system [27] typically aim for a cleaner separation of these issues. Interpreting Euclid's axioms in the spirit of this more modern approach, axioms are consistent with either infinite or finite space as in elliptic geometry , and all five axioms are consistent with a variety of topologies e.

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Archimedes c. Although the foundations of his work were put in place by Euclid, his work, unlike Euclid's, is believed to have been entirely original. Apollonius of Perga c. In this approach, a point on a plane is represented by its Cartesian x , y coordinates, a line is represented by its equation, and so on. In Euclid's original approach, the Pythagorean theorem follows from Euclid's axioms. In the Cartesian approach, the axioms are the axioms of algebra, and the equation expressing the Pythagorean theorem is then a definition of one of the terms in Euclid's axioms, which are now considered theorems.

In terms of analytic geometry, the restriction of classical geometry to compass and straightedge constructions means a restriction to first- and second-order equations, e. Also in the 17th century, Girard Desargues , motivated by the theory of perspective , introduced the concept of idealized points, lines, and planes at infinity. The result can be considered as a type of generalized geometry, projective geometry , but it can also be used to produce proofs in ordinary Euclidean geometry in which the number of special cases is reduced.

Geometers of the 18th century struggled to define the boundaries of the Euclidean system. Many tried in vain to prove the fifth postulate from the first four. By , at least 28 different proofs had been published, but all were found incorrect. Leading up to this period, geometers also tried to determine what constructions could be accomplished in Euclidean geometry.

For example, the problem of trisecting an angle with a compass and straightedge is one that naturally occurs within the theory, since the axioms refer to constructive operations that can be carried out with those tools. However, centuries of efforts failed to find a solution to this problem, until Pierre Wantzel published a proof in that such a construction was impossible.

Other constructions that were proved impossible include doubling the cube and squaring the circle. In the case of doubling the cube, the impossibility of the construction originates from the fact that the compass and straightedge method involve equations whose order is an integral power of two, [32] while doubling a cube requires the solution of a third-order equation. Euler discussed a generalization of Euclidean geometry called affine geometry , which retains the fifth postulate unmodified while weakening postulates three and four in a way that eliminates the notions of angle whence right triangles become meaningless and of equality of length of line segments in general whence circles become meaningless while retaining the notions of parallelism as an equivalence relation between lines, and equality of length of parallel line segments so line segments continue to have a midpoint.

In the 19th century, it was also realized that Euclid's ten axioms and common notions do not suffice to prove all of the theorems stated in the Elements. For example, Euclid assumed implicitly that any line contains at least two points, but this assumption cannot be proved from the other axioms, and therefore must be an axiom itself. The very first geometric proof in the Elements, shown in the figure above, is that any line segment is part of a triangle; Euclid constructs this in the usual way, by drawing circles around both endpoints and taking their intersection as the third vertex.

His axioms, however, do not guarantee that the circles actually intersect, because they do not assert the geometrical property of continuity, which in Cartesian terms is equivalent to the completeness property of the real numbers. Starting with Moritz Pasch in , many improved axiomatic systems for geometry have been proposed, the best known being those of Hilbert , [35] George Birkhoff , [36] and Tarski. Einstein's theory of special relativity involves a four-dimensional space-time , the Minkowski space , which is non-Euclidean. This shows that non-Euclidean geometries, which were introduced a few years earlier for showing that the parallel postulate cannot be proved, are also useful for describing the physical world.

However, the three-dimensional "space part" of the Minkowski space remains the space of Euclidean geometry. This is not the case with general relativity , for which the geometry of the space part of space-time is not Euclidean geometry.

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A relatively weak gravitational field, such as the Earth's or the sun's, is represented by a metric that is approximately, but not exactly, Euclidean. Until the 20th century, there was no technology capable of detecting the deviations from Euclidean geometry, but Einstein predicted that such deviations would exist.

They were later verified by observations such as the slight bending of starlight by the Sun during a solar eclipse in , and such considerations are now an integral part of the software that runs the GPS system. Euclid sometimes distinguished explicitly between "finite lines" e. However, he typically did not make such distinctions unless they were necessary.

The postulates do not explicitly refer to infinite lines, although for example some commentators interpret postulate 3, existence of a circle with any radius, as implying that space is infinite. The notion of infinitesimal quantities had previously been discussed extensively by the Eleatic School , but nobody had been able to put them on a firm logical basis, with paradoxes such as Zeno's paradox occurring that had not been resolved to universal satisfaction.

Euclid used the method of exhaustion rather than infinitesimals. Later ancient commentators, such as Proclus — CE , treated many questions about infinity as issues demanding proof and, e. At the turn of the 20th century, Otto Stolz , Paul du Bois-Reymond , Giuseppe Veronese , and others produced controversial work on non-Archimedean models of Euclidean geometry, in which the distance between two points may be infinite or infinitesimal, in the Newton — Leibniz sense.

One reason that the ancients treated the parallel postulate as less certain than the others is that verifying it physically would require us to inspect two lines to check that they never intersected, even at some very distant point, and this inspection could potentially take an infinite amount of time.

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The modern formulation of proof by induction was not developed until the 17th century, but some later commentators consider it implicit in some of Euclid's proofs, e. Supposed paradoxes involving infinite series, such as Zeno's paradox , predated Euclid. Euclid avoided such discussions, giving, for example, the expression for the partial sums of the geometric series in IX.

Euclid frequently used the method of proof by contradiction , and therefore the traditional presentation of Euclidean geometry assumes classical logic , in which every proposition is either true or false, i. Placing Euclidean geometry on a solid axiomatic basis was a preoccupation of mathematicians for centuries. Then, the system of ideas that we have initially chosen is simply one interpretation of the undefined symbols; but.. Logical questions thus become completely independent of empirical or psychological questions The system of undefined symbols can then be regarded as the abstraction obtained from the specialized theories that result when That is, mathematics is context-independent knowledge within a hierarchical framework.

As said by Bertrand Russell : [48]. If our hypothesis is about anything , and not about some one or more particular things, then our deductions constitute mathematics. Thus, mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true. Such foundational approaches range between foundationalism and formalism. The process of abstract axiomatization as exemplified by Hilbert's axioms reduces geometry to theorem proving or predicate logic.

In contrast, the Greeks used construction postulates, and emphasized problem solving. To describe problem solving adequately requires a richer system of logical concepts.

Andrei Nicholaevich Kolmogorov proposed a problem solving basis for geometry. If proof simply follows conviction of truth rather than contributing to its construction and is only experienced as a demonstration of something already known to be true, it is likely to remain meaningless and purposeless in the eyes of students. From Wikipedia, the free encyclopedia. Mathematical system attributed to Euclid. For other uses, see Plane geometry disambiguation. Projecting a sphere to a plane. Outline History. Concepts Features. By using our site, you agree to our collection of information through the use of cookies.

To learn more, view our Privacy Policy. Log In Sign Up. Papers People. The theory of a convex quadrilateral and a circle that forms "Pascal points" - the properties of "Pascal points" on the sides of a convex quadrilateral. Euclidean geometry is one of the oldest branches of mathematics — the properties of different shapes have been investigated for thousands of years.

For this reason, many tend to believe that today it is almost impossible to discover new For this reason, many tend to believe that today it is almost impossible to discover new properties and new directions for research in Euclidean geometry.

## Euclidean geometry

Save to Library. Pascal-points quadrilaterals inscribed in a cyclic quadrilateral. This paper presents some new theorems about the Pascal points of a quadrilateral. We shall begin by explaining what these are. New points that belong to the nine-point circle. In the present paper, we show that the point of intersection of the bimedians of a cyclic quadrilateral belongs to the nine-point circle Euler's circle of the triangle with one vertex at the point of intersection of the quadrilateral's In the present paper, we show that the point of intersection of the bimedians of a cyclic quadrilateral belongs to the nine-point circle Euler's circle of the triangle with one vertex at the point of intersection of the quadrilateral's diagonals and the other vertices at the points of intersection of the extensions of its pairs of opposite sides.

We shall also show that for a given arbitrary triangle, this property permits us to obtain new points that belong to the nine-point circle of this triangle. We define the concept of a "rectangle defined by a circle that forms Pascal points and a Pascal-points circle" in an orthodiagonal quadrilateral. This rectangle is inscribed in the given orthodiagonal quadrilateral.

We show that every We show that every orthodiagonal quadrilateral has an infinite set of such rectangles. We also investigate the properties of this set of rectangles: we show that the angle between the diagonals is equal in all the rectangles and we find the rectangle with the smallest area and perimeter, and more.

We also examine the intersection of this set of rectangles with another set of rectangles inscribed in the orthodiagonal quadrilateral. This second set satisfies the condition that the sides of the rectangles are parallel to the diagonals of the given quadrilateral. Finally, we prove the uniqueness of the set of rectangles defined by circles that form Pascal points and Pascal-points circles. U of Toronto P, forthcoming.