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In [[hyperbolic geometry]], the '''law of cosines''' is a pair of theorems relating the sides and angles of triangles on a [[Hyperbolic geometry|hyperbolic plane]], analogous to the planar [[law of cosines]] from plane [[trigonometry]], or the [[spherical law of cosines]] in [[spherical trigonometry]]. | |||
Take a hyperbolic plane whose [[Gaussian curvature]] is <math>-\frac1{k^2}</math>. Then given a [[hyperbolic triangle]] ''ABC'' with angles ''α'', ''β'', ''γ'', and side lengths ''BC'' = ''a'', ''AC'' = ''b'', and ''AB'' = ''c'', the following two rules hold: | |||
:<math>\cosh\frac ak = \cosh\frac bk \cosh\frac ck - \sinh\frac bk \sinh\frac ck \cos\alpha, \,</math> | |||
considering the sides, while | |||
:<math>\cos\alpha = -\cos\beta \cos\gamma + \sin\beta \sin\gamma \cosh\frac ak, \,</math> | |||
for the angles. | |||
Christian Houzel (page 8) indicates that the hyperbolic law of cosines implies the [[angle of parallelism]] in the case of an ideal hyperbolic triangle: | |||
:When α = 0, that is when the vertex ''A'' is rejected to infinity and the sides ''BA'' and ''CA'' are ''parallel'', the first member equals 1; let us suppose in addition that γ = π/2 so that cos γ = 0 and sin γ = 1. The angle at ''B'' takes a value β given by 1 = sin β cosh(''a''/k); this angle was later called ''angle of parallelism'' and Lobachevsky noted it by ''F(a)'' or Π(''a''). | |||
==See also== | |||
* [[Law_of_sines#Hyperbolic_case|Hyperbolic law of sines]] | |||
== External links == | |||
* [http://www.math.tu-berlin.de/geometrie/Lehre/SS04/VisMath/mwiki/index.php/NonEuclideanGeometry Non Euclidean Geometry, Math Wiki at TU Berlin] | |||
==References== | |||
* Houzel, Christian (1992) "The Birth of Non-Euclidean Geometry", pages 3 to 21 in ''1830–1930: A Century of Geometry'', Lecture Notes in Physics #402, [[Springer-Verlag]] ISBN 3-540-55408-4 . | |||
* {{Cite book|last=Anderson|first=James W.|title=Hyperbolic geometry|year=2005|publisher=Springer|location=London|isbn=1-85233-934-9|edition=2nd ed.}} | |||
* [[Miles Reid]] & Balázs Szendröi (2005) ''Geometry and Topology'', §3.10 Hyperbolic triangles and trig, [[Cambridge University Press]], ISBN 0-521-61325-6, {{MathSciNet|id=2194744}}. | |||
* {{Cite book| last=Reiman | first=István | year=1999 | title=Geometria és határterületei | publisher=Szalay Könyvkiadó és Kereskedőház Kft. | isbn=978-963-237-012-5 }} | |||
[[Category:Hyperbolic geometry]] | |||
[[es:Teorema del coseno#Geometría hiperbólica]] | |||
[[fr:Théorème d'Al-Kashi#Géométrie hyperbolique]] | |||
[[pl:Twierdzenie cosinusów#Wzory cosinusów w geometriach nieeuklidesowych]] | |||
{{geometry-stub}} |
Revision as of 08:20, 30 October 2013
In hyperbolic geometry, the law of cosines is a pair of theorems relating the sides and angles of triangles on a hyperbolic plane, analogous to the planar law of cosines from plane trigonometry, or the spherical law of cosines in spherical trigonometry.
Take a hyperbolic plane whose Gaussian curvature is . Then given a hyperbolic triangle ABC with angles α, β, γ, and side lengths BC = a, AC = b, and AB = c, the following two rules hold:
considering the sides, while
for the angles.
Christian Houzel (page 8) indicates that the hyperbolic law of cosines implies the angle of parallelism in the case of an ideal hyperbolic triangle:
- When α = 0, that is when the vertex A is rejected to infinity and the sides BA and CA are parallel, the first member equals 1; let us suppose in addition that γ = π/2 so that cos γ = 0 and sin γ = 1. The angle at B takes a value β given by 1 = sin β cosh(a/k); this angle was later called angle of parallelism and Lobachevsky noted it by F(a) or Π(a).
See also
External links
References
- Houzel, Christian (1992) "The Birth of Non-Euclidean Geometry", pages 3 to 21 in 1830–1930: A Century of Geometry, Lecture Notes in Physics #402, Springer-Verlag ISBN 3-540-55408-4 .
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 - Miles Reid & Balázs Szendröi (2005) Geometry and Topology, §3.10 Hyperbolic triangles and trig, Cambridge University Press, ISBN 0-521-61325-6, Template:MathSciNet.
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
es:Teorema del coseno#Geometría hiperbólica fr:Théorème d'Al-Kashi#Géométrie hyperbolique pl:Twierdzenie cosinusów#Wzory cosinusów w geometriach nieeuklidesowych
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