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| In [[spherical trigonometry]], the '''law of cosines''' (also called the '''cosine rule for sides'''<ref name=VNR/>) is a theorem relating the sides and angles of spherical triangles, analogous to the ordinary [[law of cosines]] from plane [[trigonometry]].
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| [[Image:Law-of-haversines.svg|right|thumb|Spherical triangle solved by the law of cosines.]]
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| Given a unit sphere, a "spherical triangle" on the surface of the sphere is defined by the [[great circle]]s connecting three points '''u''', '''v''', and '''w''' on the sphere (shown at right). If the lengths of these three sides are ''a'' (from '''u''' to '''v'''), ''b'' (from '''u''' to '''w'''), and ''c'' (from '''v''' to '''w'''), and the angle of the corner opposite ''c'' is ''C'', then the (first) spherical law of cosines states:<ref name=Ireneus>Romuald Ireneus 'Scibor-Marchocki, [http://www.webcitation.org/query?url=http://www.geocities.com/ResearchTriangle/2363/trig02.html&date=2009-10-25+09:44:36 Spherical trigonometry], ''Elementary-Geometry Trigonometry'' web page (1997).</ref><ref name=VNR>W. Gellert, S. Gottwald, M. Hellwich, H. Kästner, and H. Küstner, ''The VNR Concise Encyclopedia of Mathematics'', 2nd ed., ch. 12 (Van Nostrand Reinhold: New York, 1989).</ref>
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| :<math>\cos(c) = \cos(a) \cos(b) + \sin(a) \sin(b) \cos(C). \,</math>
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| Since this is a unit sphere, the lengths ''a'', ''b'', and ''c'' are simply equal to the angles (in [[radian]]s) subtended by those sides from the center of the sphere (for a non-unit sphere, they are the distances divided by the radius). As a special case, for <math>C = \pi/2 </math>, then <math>\cos(C) =0 \,</math> and one obtains the spherical analogue of the [[Pythagorean theorem]]:
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| :<math>\cos(c) = \cos(a) \cos(b). \,</math>
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| A variation on the law of cosines, the second spherical law of cosines,<ref>{{Cite book| last=Reiman | first=István | year=1999 | title=Geometria és határterületei | publisher=Szalay Könyvkiadó és Kereskedőház Kft. | page=83 }}</ref> (also called the '''cosine rule for angles'''<ref name=VNR/>) states:
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| :<math>\cos(A) = -\cos(B)\cos(C) + \sin(B)\sin(C)\cos(a) \,</math>
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| where ''A'' and ''B'' are the angles of the corners opposite to sides ''a'' and ''b'', respectively. It can be obtained from consideration of a spherical triangle dual to the given one.
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| If the law of cosines is used to solve for ''c'', the necessity of inverting the cosine magnifies [[rounding error]]s when ''c'' is small. In this case, the alternative formulation of the [[law of haversines]] is preferable.<ref>R. W. Sinnott, "Virtues of the Haversine", Sky and Telescope 68 (2), 159 (1984).</ref>
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| ==Proof==
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| A proof of the law of cosines can be constructed as follows.<ref name=Ireneus/> Let '''u''', '''v''', and '''w''' denote the [[unit vector]]s from the center of the sphere to those corners of the triangle. Then, the lengths (angles) of the sides are given by the [[dot product]]s:
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| :<math>\cos(a) = \mathbf{u} \cdot \mathbf{v}</math>
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| :<math>\cos(b) = \mathbf{u} \cdot \mathbf{w}</math>
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| :<math>\cos(c) = \mathbf{v} \cdot \mathbf{w}</math>
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| To get the angle ''C'', we need the [[tangent]] vectors '''t'''<sub>''a''</sub> and '''t'''<sub>''b''</sub> at '''u''' along the directions of sides ''a'' and ''b'', respectively. For example, the tangent vector '''t'''<sub>''a''</sub> is the unit vector [[perpendicular]] to '''u''' in the '''u'''-'''v''' plane, whose direction is given by the component of '''v''' perpendicular to '''u'''. This means:
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| :<math>\mathbf{t}_a = \frac{\mathbf{v} - \mathbf{u} (\mathbf{u} \cdot \mathbf{v})}{\left| \mathbf{v} - \mathbf{u} (\mathbf{u} \cdot \mathbf{v}) \right|} = \frac{\mathbf{v} - \mathbf{u} \cos(a)}{\sin(a)}</math> | |
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| where for the denominator we have used the [[Pythagorean trigonometric identity|Pythagorean identity]] sin<sup>2</sup>(''a'') = 1 − cos<sup>2</sup>(''a''). Similarly,
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| :<math>\mathbf{t}_b = \frac{\mathbf{w} - \mathbf{u} \cos(b)}{\sin(b)}.</math>
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| Then, the angle ''C'' is given by:
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| :<math>\cos(C) = \mathbf{t}_a \cdot \mathbf{t}_b = \frac{\cos(c) - \cos(a) \cos(b)}{\sin(a) \sin(b)}</math>
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| from which the law of cosines immediately follows.
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| ==Proof without vectors==
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| To the diagram above, add a plane tangent to the sphere at '''u''', and extend radii from the center of the sphere '''O''' through '''v''' and through '''w''' to meet the plane at points '''y''' and '''z'''. We then have two plane triangles with a side in common: the triangle containing '''u''', '''y''' and '''z''' and the one containing '''O''', '''y''' and '''z'''. Sides of the first triangle are tan a and tan b, with angle C between them; sides of the second triangle are sec a and sec b, with angle c between them. By the [[law of cosines]] for plane triangles (and remembering that <math>\sec^2</math> of any angle is <math>\tan^2 + 1</math>),
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| <math> \tan^2 a + \tan^2 b - 2\tan a \tan b \cos C \;\;\;=\;\;\; \sec^2 a + \sec^2 b - 2 \sec a \sec b \cos c</math>
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| <math>\;\;= \;\; 2 + \tan^2 a + \tan^2 b - 2 \sec a \sec b \cos c </math>
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| So
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| <math> - \tan a \tan b \cos C \;= \; 1 - \sec a \sec b \cos c</math>
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| Multiply both sides by <math> \cos a \cos b </math> and rearrange.
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| == Planar limit: small angles ==
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| For ''small'' spherical triangles, i.e. for small ''a'', ''b'', and ''c'', the spherical law of cosines is approximately the same as the ordinary planar law of cosines,
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| : <math>c^2 \approx a^2 + b^2 - 2ab\cos(C) . \,\!</math>
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| To prove this, we'll use the [[small-angle approximation]] obtained from the [[Maclaurin series]] for the cosine and sine functions:
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| : <math>\cos(a) = 1 - \frac{a^2}{2} + O(a^4),\, \sin(a) = a + O(a^3)</math>
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| Substituting these expressions into the spherical law of cosines nets:
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| : <math>1 - \frac{c^2}{2} + O(c^4) = 1 - \frac{a^2}{2} - \frac{b^2}{2} + \frac{a^2 b^2}{4} + O(a^4) + O(b^4) + \cos(C)(ab + O(a^3 b) + O(ab ^ 3) + O(a^3 b^3))</math>
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| or after simplifying:
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| : <math>c^2 = a^2 + b^2 - 2ab\cos(C) + O(c^4) + O(a^4) + O(b^4) + O(a^2 b^2) + O(a^3 b) + O(ab ^ 3) + O(a^3 b^3).</math>
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| Remembering the properties of [[big O notation]], we can discard summands where the lowest exponent for ''a'' or ''b'' is greater than 1, so finally, the error in this approximation is:
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| : <math>O(c^4) + O(a^3 b) + O(a b^3) . \,\!</math>
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| == See also ==
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| * [[Half-side formula]]
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| * [[Hyperbolic law of cosines]]
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| * [[Solution of triangles]]
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| ==Notes==
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| <references/>
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| [[Category:Spherical trigonometry]]
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| [[Category:Articles containing proofs]]
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| [[Category:Theorems in geometry]]
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| [[he:טריגונומטריה ספירית#משפט הקוסינוסים]]
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The author is called Irwin. For many years he's been living in North Dakota and his family members enjoys it. She is a librarian but she's always needed her personal company. The favorite pastime for my children and me is to play baseball and I'm attempting to make it a profession.
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