Line (geometry) | Wikipedia audio article
Channel: wikipedia tts
Date: 2019-10-04

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Line_(geometry)


00:01:53 1 Definitions versus descriptions
00:03:25 2 In Euclidean geometry
00:04:53 2.1 On the Cartesian plane
00:14:16 2.1.1 In normal form
00:16:02 2.2 In polar coordinates
00:16:19 2.3 As a vector equation
00:19:27 2.4 In Euclidean space
00:20:30 2.4.1 Collinear points
00:21:32 2.5 Types of lines
00:21:50 3 In projective geometry
00:23:25 4 Extensions
00:23:54 4.1 Ray
00:25:47 4.2 Line segment
00:26:33 4.3 Geodesics
00:26:41 5 See also
00:28:40 6 Notes
00:29:13 7 References
00:29:36 8 External links



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SUMMARY
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The notion of line or straight line was introduced by ancient mathematicians to represent straight objects (i.e., having no curvature) with negligible width and depth. Lines are an idealization of such objects. Until the 17th century, lines were defined as the "[…] first species of quantity, which has only one dimension, namely length, without any width nor depth, and is nothing else than the flow or run of the point which […] will leave from its imaginary moving some vestige in length, exempt of any width. […] The straight line is that which is equally extended between its points."Euclid described a line as "breadthless length" which "lies equally with respect to the points on itself"; he introduced several postulates as basic unprovable properties from which he constructed all of geometry, which is now called Euclidean geometry to avoid confusion with other geometries which have been introduced since the end of the 19th century (such as non-Euclidean, projective and affine geometry).
In modern mathematics, given the multitude of geometries, the concept of a line is closely tied to the way the geometry is described. For instance, in analytic geometry, a line in the plane is often defined as the set of points whose coordinates satisfy a given linear equation, but in a more abstract setting, such as incidence geometry, a line may be an independent object, distinct from the set of points which lie on it.
When a geometry is described by a set of axioms, the notion of a line is usually left undefined (a so-called primitive object). The properties of lines are then determined by the axioms which refer to them. One advantage to this approach is the flexibility it gives to users of the geometry.  Thus in differential geometry a line may be interpreted as a geodesic (shortest path between points), while in some projective geometries a line is a 2-dimensional vector space (all linear combinations of two independent vectors). This flexibility also extends beyond mathematics and, for example, permits physicists to think of the path of a light ray as being a line.