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Solution of triangles (Template:Lang-lat) is the historical term for solving the main trigonometric problem of finding the characteristics of a triangle (angles and lengths of sides), when some of these are known. The triangle can be located on a plane or on a sphere. Applications requiring triangle solutions include geodesy, astronomy, construction, and navigation.

Solving plane triangles

A general form triangle has six main characteristics (see picture): three linear (side lengths $a, b, c$ ) and three angular ( $α, β, γ$ ). The classical plane trigonometry problem is to specify three of the six characteristics and determine the other three. A triangle can be uniquely determined in this sense when given any of the following:^[1]^[2]

Three sides (SSS)
Two sides and the included angle (SAS)
Two sides and an angle not included between them (SSA), if the side length adjacent to the angle is shorter than the other side length.
A side and the two angles adjacent to it (ASA)
A side, the angle opposite to it and an angle adjacent to it (AAS).
Three angles (AAA) on the sphere (but not in the plane).

For all cases in the plane, at least one of the side lengths must be specified. If only the angles are given, the side lengths cannot be determined, because any similar triangle is a solution.

Main theorems

The standard method of solving the problem is to use fundamental relations.

Law of cosines: $a^{2}=b^{2}+c^{2}-2bc\cos \alpha$; $b^{2}=a^{2}+c^{2}-2ac\cos \beta$; $c^{2}=a^{2}+b^{2}-2ab\cos \gamma$
Law of sines: ${\frac {a}{\sin \alpha }}={\frac {b}{\sin \beta }}={\frac {c}{\sin \gamma }}$
Sum of angles: $\alpha +\beta +\gamma =180^{\circ }$
Law of tangents: ${\frac {a-b}{a+b}}={\frac {\mathrm {tan} [{\frac {1}{2}}(\alpha -\beta )]}{\mathrm {tan} [{\frac {1}{2}}(\alpha +\beta )]}}.$

There are other (sometimes practically useful) universal relations: the law of cotangents and Mollweide's formula.

Notes

To find an unknown angle, the law of cosines is safer than the law of sines. The reason is that the value of sine for the angle of the triangle does not uniquely determine this angle. For example, if $sin β = 0.5$ , the angle $β$ can equal either 30° or 150°. Using the law of cosines avoids this problem: within the interval from 0° to 180° the cosine value unambiguously determines its angle. On the other hand, if the angle is small (or close to 180°), then it is more robust numerically to determine it from its sine than its cosine because the arc-cosine function has a divergent derivative at 1 (or −1).
We assume that the relative position of specified characteristics is known. If not, the mirror reflection of the triangle will also be a solution. For example, three side lengths uniquely define either a triangle or its reflection.

Three sides given (SSS)

Let three side lengths $a, b, c$ be specified. To find the angles $α, β$ , the law of cosines can be used:^[3]

\alpha =\arccos {\frac {b^{2}+c^{2}-a^{2}}{2bc}}

\beta =\arccos {\frac {a^{2}+c^{2}-b^{2}}{2ac}}.

Then angle $γ = 180° - α - β$ .

Some sources recommend to find angle $β$ from the law of sines but (as Note 1 above states) there is a risk of confusing an acute angle value with an obtuse one.

Another method of calculating the angles from known sides is to apply the law of cotangents.

Two sides and the included angle given (SAS)

Here the lengths of sides $a, b$ and the angle $γ$ between these sides are known. The third side can be determined from the law of cosines:^[4]

c={\sqrt {a^{2}+b^{2}-2ab\cos \gamma }}.

Now we use law of cosines to find the second angle:

\alpha =\arccos {\frac {b^{2}+c^{2}-a^{2}}{2bc}}.

Finally, $β = 180° - α - γ$ .

Two sides and non-included angle given (SSA)

This case is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length. Assume that two sides $b, c$ and the angle $β$ are known. The equation for the angle $γ$ can be implied from the law of sines:^[5]

\sin \gamma ={\frac {c}{b}}\sin \beta .

We denote further $D = .mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num{display:block;line-height:1em;margin:0.0em 0.1em;border-bottom:1px solid}.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0.1em 0.1em}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}⁠c/b⁠ sin β$ (the equation's right side). There are four possible cases:

If $D > 1$ , no such triangle exists because the side $b$ does not reach line $BC$ . For the same reason a solution does not exist if the angle $β \geq 90°$ and $b \leq c$ .
If $D = 1$ , a unique solution exists: $γ = 90°$ , i.e., the triangle is right-angled.

If D < 1 two alternatives are possible.
1. If $b < c$ , the angle $γ$ may be acute: $γ = arcsin D$ or obtuse: $γ' = 180° - γ$ . The picture on right shows the point $C$ , the side $b$ and the angle $γ$ as the first solution, and the point $C'$ , side $b'$ and the angle $γ'$ as the second solution.
2. If $b \geq c$ then $β \geq γ$ (the larger side corresponds to a larger angle). Since no triangle can have two obtuse angles, $γ$ is an acute angle and the solution $γ = arcsin D$ is unique.

Once $γ$ is obtained, the third angle $α = 180° - β - γ$ .

The third side can then be found from the law of sines:

a=b\ {\frac {\sin \alpha }{\sin \beta }}

A side and two adjacent angles given (ASA)

The known characteristics are the side $c$ and the angles $α, β$ . The third angle $γ = 180° - α - β$ .

Two unknown side can be calculated from the law of sines:^[6]

a=c\ {\frac {\sin \alpha }{\sin \gamma }};\quad b=c\ {\frac {\sin \beta }{\sin \gamma }}.

A side, one adjacent angle and the opposite angle given (AAS)

The procedure for solving an AAS triangle is same as that for an ASA triangle: First, find the third angle by using the angle sum property of a triangle, then find the other two sides using the law of sines.

Solving spherical triangles

The general spherical triangle is fully determined by three of its six characteristics (3 sides and 3 angles). Note that the sides $a, b, c$ of a spherical triangle are measured by angular rather than linear units, based on the corresponding central angles.

The solution of triangles for non-Euclidean spherical geometry has some differences from the planar case. For example, the sum of the three angles $α + β + γ$ depends on the triangle. In addition, there are no unequal similar triangles, and so the problem of constructing a triangle with specified three angles has a unique solution. The basic relations used to solve a problem are similar to those of the planar case: see Law of cosines (spherical) and Law of sines (spherical).

Among other relationships that may be useful are the half-side formula and Napier's analogies:^[7]

$\tan {\frac {c}{2}}\cos {\frac {\alpha -\beta }{2}}=\tan {\frac {a+b}{2}}\cos {\frac {\alpha +\beta }{2}}$
$\tan {\frac {c}{2}}\sin {\frac {\alpha -\beta }{2}}=\tan {\frac {a-b}{2}}\sin {\frac {\alpha +\beta }{2}}$
$\cot {\frac {\gamma }{2}}\cos {\frac {a-b}{2}}=\tan {\frac {\alpha +\beta }{2}}\cos {\frac {a+b}{2}}$
$\cot {\frac {\gamma }{2}}\sin {\frac {a-b}{2}}=\tan {\frac {\alpha -\beta }{2}}\sin {\frac {a+b}{2}}.$

Three sides given (spherical SSS)

Known: the sides $a, b, c$ (in angular units). The triangle's angles are computed from the spherical law of cosines:

\alpha =\arccos \left({\frac {\cos a-\cos b\ \cos c}{\sin b\ \sin c}}\right),

\beta =\arccos \left({\frac {\cos b-\cos c\ \cos a}{\sin c\ \sin a}}\right),

\gamma =\arccos \left({\frac {\cos c-\cos a\ \cos b}{\sin a\ \sin b}}\right).

Two sides and the included angle given (spherical SAS)

Known: the sides $a, b$ and the angle $γ$ between them. The side $c$ can be found from the law of cosines:

c=\arccos \left(\cos a\cos b+\sin a\sin b\cos \gamma \right).

The angles $α', β$ can be calculated as above, or by using Napier's analogies:

\alpha =\arctan \ {\frac {2\sin a}{\tan({\frac {\gamma }{2}})\sin(b+a)+\cot({\frac {\gamma }{2}})\sin(b-a)}},

\beta =\arctan \ {\frac {2\sin b}{\tan({\frac {\gamma }{2}})\sin(a+b)+\cot({\frac {\gamma }{2}})\sin(a-b)}}.

This problem arises in the navigation problem of finding the great circle between two points on the earth specified by their latitude and longitude; in this application, it is important to use formulas which are not susceptible to round-off errors. For this purpose, the following formulas (which may be derived using vector algebra) can be used:

{\begin{aligned}c&=\arctan {\frac {\sqrt {(\sin a\cos b-\cos a\sin b\cos \gamma )^{2}+(\sin b\sin \gamma )^{2}}}{\cos a\cos b+\sin a\sin b\cos \gamma }},\\\alpha &=\arctan {\frac {\sin a\sin \gamma }{\sin b\cos a-\cos b\sin a\cos \gamma }},\\\beta &=\arctan {\frac {\sin b\sin \gamma }{\sin a\cos b-\cos a\sin b\cos \gamma }},\end{aligned}}

where the signs of the numerators and denominators in these expressions should be used to determine the quadrant of the arctangent.

Two sides and non-included angle given (spherical SSA)

This problem is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length. Known: the sides $b, c$ and the angle $β$ not between them. A solution exists if the following condition holds:

b>\arcsin(\sin c\,\sin \beta ).

The angle $γ$ can be found from the spherical law of sines:

\gamma =\arcsin \left({\frac {\sin c\,\sin \beta }{\sin b}}\right).

As for the plane case, if $b < c$ then there are two solutions: $γ$ and $180° - γ$ .

We can find other characteristics by using Napier's analogies:

a=2\arctan \left[\tan \left({\tfrac {1}{2}}(b-c)\right){\frac {\sin \left({\tfrac {1}{2}}(\beta +\gamma )\right)}{\sin \left({\tfrac {1}{2}}(\beta -\gamma )\right)}}\right],

\alpha =2\operatorname {arccot} \left[\tan \left({\tfrac {1}{2}}(\beta -\gamma )\right){\frac {\sin \left({\tfrac {1}{2}}(b+c)\right)}{\sin \left({\tfrac {1}{2}}(b-c)\right)}}\right].

A side and two adjacent angles given (spherical ASA)

Known: the side $c$ and the angles $α, β$ . First we determine the angle $γ$ using the law of cosines:

\gamma =\arccos(\sin \alpha \sin \beta \cos c-\cos \alpha \cos \beta ).\,

We can find the two unknown sides from the law of cosines (using the calculated angle $γ$ ):

a=\arccos \left({\frac {\cos \alpha +\cos \beta \cos \gamma }{\sin \beta \sin \gamma }}\right),

b=\arccos \left({\frac {\cos \beta +\cos \gamma \cos \alpha }{\sin \gamma \sin \alpha }}\right),

or by using Napier's analogies:

a=\arctan \left[{\frac {2\sin \alpha }{\cot({\frac {c}{2}})\sin(\beta +\alpha )+\tan({\frac {c}{2}})\sin(\beta -\alpha )}}\right],

b=\arctan \left[{\frac {2\sin \beta }{\cot({\frac {c}{2}})\sin(\alpha +\beta )+\tan({\frac {c}{2}})\sin(\alpha -\beta )}}\right].

A side, one adjacent angle and the opposite angle given (spherical AAS)

Known: the side $a$ and the angles $α, β$ . The side $b$ can be found from the law of sines:

b=\arcsin \left({\frac {\sin a\,\sin \beta }{\sin \alpha }}\right).

If the angle for the side $a$ is acute and $α > β$ , another solution exists:

b=\pi -\arcsin \left({\frac {\sin a\,\sin \beta }{\sin \alpha }}\right).

We can find other characteristics by using Napier's analogies:

c=2\arctan \left[\tan \left({\tfrac {1}{2}}(a-b)\right){\frac {\sin \left({\tfrac {1}{2}}(\alpha +\beta )\right)}{\sin \left({\frac {1}{2}}(\alpha -\beta )\right)}}\right],

\gamma =2\operatorname {arccot} \left[\tan \left({\tfrac {1}{2}}(\alpha -\beta )\right){\frac {\sin \left({\tfrac {1}{2}}(a+b)\right)}{\sin \left({\frac {1}{2}}(a-b)\right)}}\right].

Three angles given (spherical AAA)

Known: the angles $α, β, γ$ . From the law of cosines we infer:

a=\arccos \left({\frac {\cos \alpha +\cos \beta \cos \gamma }{\sin \beta \sin \gamma }}\right),

b=\arccos \left({\frac {\cos \beta +\cos \gamma \cos \alpha }{\sin \gamma \sin \alpha }}\right),

c=\arccos \left({\frac {\cos \gamma +\cos \alpha \cos \beta }{\sin \alpha \sin \beta }}\right).

Solving right-angled spherical triangles

The above algorithms become much simpler if one of the angles of a triangle (for example, the angle $C$ ) is the right angle. Such a spherical triangle is fully defined by its two elements, and the other three can be calculated using Napier's Pentagon or the following relations.

\sin a=\sin c\cdot \sin A

(from the Law of sines (spherical))

\tan a=\sin b\cdot \tan A

\cos c=\cos a\cdot \cos b

(from the law of cosines (spherical))

\tan b=\tan c\cdot \cos A

\cos A=\cos a\cdot \sin B

(also from the law of cosines)

\cos c=\cot A\cdot \cot B

Some applications

Triangulation

If one wants to measure the distance $d$ from shore to a remote ship via triangulation, one marks on the shore two points with known distance $l$ between them (the baseline). Let $α, β$ be the angles between the baseline and the direction to the ship.

From the formulae above (ASA case) one can define the length of the triangle height:

d={\frac {\sin \alpha \,\sin \beta }{\sin(\alpha +\beta )}}\,l={\frac {\tan \alpha \,\tan \beta }{\tan \alpha +\tan \beta }}\,l.

This method is used in cabotage. The angles $α, β$ are defined by observation of familiar landmarks from the ship.

As another example, if one wants to measure the height $h$ of a mountain or a high building, the angles $α, β$ from two ground points to the top are specified. Let $l$ be the distance between these points. From the same ASA case formulas we obtain:

h={\frac {\sin \alpha \,\sin \beta }{\sin(\beta -\alpha )}}\,l={\frac {\tan \alpha \,\tan \beta }{\tan \beta -\tan \alpha }}\,l.

The distance between two points on the globe

To calculate the distance between two points on the globe,

Point A: latitude

λ A

, longitude

L A

, and

Point B: latitude

λ B

, longitude

L B

we consider the spherical triangle $ABC$ , where $C$ is the North Pole. Some characteristics are:

a=90^{\mathrm {o} }-\lambda _{\mathrm {B} },\,

b=90^{\mathrm {o} }-\lambda _{\mathrm {A} },\,

\gamma =L_{\mathrm {A} }-L_{\mathrm {B} }.\,

If two sides and the included angle given, we obtain from the formulas

\mathrm {AB} =R\arccos \left[\sin \lambda _{\mathrm {A} }\,\sin \lambda _{\mathrm {B} }+\cos \lambda _{\mathrm {A} }\,\cos \lambda _{\mathrm {B} }\,\cos \left(L_{\mathrm {A} }-L_{\mathrm {B} }\right)\right].

Here $R$ is the Earth's radius.

References

^ "Solving Triangles". Maths is Fun. Retrieved 4 April 2012.
^ "Solving Triangles". web.horacemann.org. Retrieved 4 April 2012.
^ "Solving SSS Triangles". Maths is Fun. Retrieved 13 January 2015.
^ "Solving SAS Triangles". Maths is Fun. Retrieved 13 January 2015.
^ "Solving SSA Triangles". Maths is Fun. Retrieved 9 March 2013.
^ "Solving ASA Triangles". Maths is Fun. Retrieved 13 January 2015.
^ Napier's Analogies at MathWorld

Euclid (1956) [1925]. Sir Thomas Heath (ed.). The Thirteen Books of the Elements. Volume I. Translated with introduction and commentary. Dover. ISBN 0-486-60088-2.

External links

Trigonometric Delights, by Eli Maor, Princeton University Press, 1998. Ebook version, in PDF format, full text presented.
Trigonometry by Alfred Monroe Kenyon and Louis Ingold, The Macmillan Company, 1914. In images, full text presented. Google book.
Spherical trigonometry on Math World.
Intro to Spherical Trig. Includes discussion of The Napier circle and Napier's rules
Spherical Trigonometry — for the use of colleges and schools by I. Todhunter, M.A., F.R.S. Historical Math Monograph posted by Cornell University Library.
Triangulator – Triangle solver. Solve any plane triangle problem with the minimum of input data. Drawing of the solved triangle.
TriSph – Free software to solve the spherical triangles, configurable to different practical applications and configured for gnomonic.
Spherical Triangle Calculator – Solves spherical triangles.

[1] "Solving Triangles". Maths is Fun. Retrieved 4 April 2012.

[2] "Solving Triangles". web.horacemann.org. Retrieved 4 April 2012.

[3] "Solving SSS Triangles". Maths is Fun. Retrieved 13 January 2015.

[4] "Solving SAS Triangles". Maths is Fun. Retrieved 13 January 2015.

[5] "Solving SSA Triangles". Maths is Fun. Retrieved 9 March 2013.

[6] "Solving ASA Triangles". Maths is Fun. Retrieved 13 January 2015.

[7] Napier's Analogies at MathWorld

[1]

[2]

[3]

[4]

[5]

[6]

[7]

@@ Line 2: / Line 2: @@
 == Solving plane triangles ==
-[[File:Triangle - angles, vertices, sides.svg|thumb|220px|right|<center>Standard notation in the triangle</center>]]
+[[File:Triangle - angles, vertices, sides.svg|thumb|220px|right|<center>Standard notation for a triangle</center>]]
-A general form triangle has six main characteristics (see picture): three linear (side lengths <math>~a,b,c</math>) and three angular (<math>~\alpha,\beta,\gamma</math>). The classical plane trigonometry problem is to specify three of the six characteristics and  determine the other three.  A triangle can be uniquely determined in this sense when given any of the following:<ref>{{cite web | url=http://www.mathsisfun.com/algebra/trig-solving-triangles.html | title=Solving Triangles | publisher=Maths is Fun | accessdate=4 April 2012<!-- 6:37 (UTC)-->}}</ref><ref>{{cite web | url=http://web.horacemann.org/academics/math/pcbch/trig/triangle.html | title=Solving Triangles | publisher=web.horacemann.org | accessdate=4 April 2012<!-- 6:42 (UTC)-->}}</ref>
+A general form triangle has six main characteristics (see picture): three linear (side lengths {{math|''a'', ''b'', ''c''}}) and three angular ({{math|''α'', ''β'', ''γ''}}). The classical plane trigonometry problem is to specify three of the six characteristics and  determine the other three.  A triangle can be uniquely determined in this sense when given any of the following:<ref>{{cite web | url=http://www.mathsisfun.com/algebra/trig-solving-triangles.html | title=Solving Triangles | publisher=Maths is Fun | accessdate=4 April 2012<!-- 6:37 (UTC)-->}}</ref><ref>{{cite web | url=http://web.horacemann.org/academics/math/pcbch/trig/triangle.html | title=Solving Triangles | publisher=web.horacemann.org | accessdate=4 April 2012<!-- 6:42 (UTC)-->}}</ref>
 * Three sides ('''SSS''')
@@ Line 30: / Line 30: @@
 === Notes ===
-# To find an unknown angle, the [[law of cosines]] is safer than the [[law of sines]]. The reason is that the value of [[sine]] for the angle of the triangle does not uniquely determine this angle. For example, if <math>\sin \beta = 0.5</math>, the angle <math>\beta</math> can be equal either <math>30^\circ</math> or <math>150^\circ</math>. Using the law of cosines avoids this problem: within the interval from <math>0^\circ</math> to <math>180^\circ</math> the cosine value unambiguously determines its angle.  On the other hand, if the angle is small (or close to 180°), then it is more robust numerically to determine it from its sine than its cosine because the arc-cosine function has a divergent derivative at 1 (or −1).
+# To find an unknown angle, the [[law of cosines]] is safer than the [[law of sines]]. The reason is that the value of [[sine]] for the angle of the triangle does not uniquely determine this angle. For example, if {{math|sin ''β'' {{=}} 0.5}}, the angle {{math|''β''}} can equal either 30° or 150°. Using the law of cosines avoids this problem: within the interval from 0° to 180° the cosine value unambiguously determines its angle. On the other hand, if the angle is small (or close to 180°), then it is more robust numerically to determine it from its sine than its cosine because the arc-cosine function has a divergent derivative at 1 (or −1).
 # We assume that the relative position of specified characteristics is known. If not, the mirror reflection of the triangle will also be a solution. For example, three side lengths uniquely define either a triangle or its reflection.
@@ Line 36: / Line 36: @@
 === Three sides given (SSS) ===
-Let three side lengths <math>a, b, c</math> be specified. To find the angles <math>\alpha, \beta</math>, the [[law of cosines]] can be used:<ref>{{cite web |url=http://www.mathsisfun.com/algebra/trig-solving-sss-triangles.html |title=Solving SSS Triangles |publisher=Maths is Fun |accessdate=13 January 2015 }}</ref>
+Let three side lengths {{math|''a'', ''b'', ''c''}} be specified. To find the angles {{math|''α'', ''β''}}, the [[law of cosines]] can be used:<ref>{{cite web |url=http://www.mathsisfun.com/algebra/trig-solving-sss-triangles.html |title=Solving SSS Triangles |publisher=Maths is Fun |accessdate=13 January 2015 }}</ref>
 : <math> \alpha =  \arccos \frac{b^2 + c^2 - a^2} {2 b c}</math>
 : <math> \beta =  \arccos \frac{a^2 + c^2 - b^2} {2 a c}.</math>
-Then angle <math>\gamma = 180^\circ - \alpha - \beta</math>.
+Then angle {{math|''γ'' {{=}} 180° − ''α'' − ''β''}}.
-Some sources recommend to find angle <math>\beta</math> from the [[law of sines]] but (as Note 1 above states) there is a risk of confusing an acute angle value with an obtuse one.
+Some sources recommend to find angle {{math|''β''}} from the [[law of sines]] but (as Note 1 above states) there is a risk of confusing an acute angle value with an obtuse one.
 Another method of calculating the angles from known sides is to apply the [[law of cotangents]].
@@ Line 48: / Line 48: @@
 === Two sides and the included angle given (SAS) ===
-Here the lengths of sides <math>a, b</math> and the angle <math>\gamma</math> between these sides are known. The third side can be determined from the law of cosines:<ref>{{cite web |url=http://www.mathsisfun.com/algebra/trig-solving-sas-triangles.html |title=Solving SAS Triangles |publisher=Maths is Fun |accessdate=13 January 2015}}</ref>
+Here the lengths of sides {{math|''a'', ''b''}} and the angle {{math|''γ''}} between these sides are known. The third side can be determined from the law of cosines:<ref>{{cite web |url=http://www.mathsisfun.com/algebra/trig-solving-sas-triangles.html |title=Solving SAS Triangles |publisher=Maths is Fun |accessdate=13 January 2015}}</ref>
 : <math>c = \sqrt{a^2+b^2-2ab\cos\gamma}.</math>
 Now we use law of cosines to find the second angle:
 : <math> \alpha =  \arccos \frac{b^2 + c^2 - a^2} {2 b c}.</math>
-Finally, <math>\beta = 180^\circ - \alpha - \gamma.</math>
+Finally, {{math|''β'' {{=}} 180° − ''α'' − ''γ''}}.
-[[File:resolve triangle with b c beta.png|thumb|right|250px|<center>Two sides and non-included angle given</center>]]
+[[File:resolve triangle with b c beta.png|thumb|right|250px|<center>Two sides and a non-included angle given</center>]]
 === Two sides and non-included angle given (SSA) ===
-This case is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length.  Assume that two sides <math>b, c</math> and the angle <math>\beta</math> are known. The equation for the angle <math>\gamma</math> can be implied  from the [[law of sines]]:<ref>{{cite web | url=http://www.mathsisfun.com/algebra/trig-solving-ssa-triangles.html | title=Solving SSA Triangles | publisher=Maths is Fun | accessdate=9 March 2013 }}</ref>
+This case is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length.  Assume that two sides {{math|''b'', ''c''}} and the angle {{math|''β''}} are known. The equation for the angle {{math|''γ''}} can be implied  from the [[law of sines]]:<ref>{{cite web | url=http://www.mathsisfun.com/algebra/trig-solving-ssa-triangles.html | title=Solving SSA Triangles | publisher=Maths is Fun | accessdate=9 March 2013 }}</ref>
 : <math>\sin\gamma = \frac{c}{b} \sin\beta.</math>
-We denote further <math>~D=\frac{c}{b} \sin\beta</math> (equation's right side). There are four possible cases:
+We denote further {{math|''D'' {{=}} {{sfrac|''c''|''b''}} sin ''β''}} (the equation's right side). There are four possible cases:
-# If <math>D>1</math>, no such triangle exists because the side <math>b</math> does not reach line BC. For the same reason a solution does not exist if the angle <math>\beta \geqslant 90^\circ</math> and <math>b \leqslant c.</math>
+# If {{math|''D'' > 1}}, no such triangle exists because the side {{math|''b''}} does not reach line {{math|''BC''}}. For the same reason a solution does not exist if the angle {{math|''β'' ≥ 90°}} and {{math|''b'' ≤ ''c''}}.
-# If <math>D=1</math>, a unique solution exists: <math>\gamma = 90^\circ</math>, i.e., the triangle is [[Right triangle|right-angled]].
+# If {{math|''D'' {{=}} 1}}, a unique solution exists: {{math|''γ'' {{=}} 90°}}, i.e., the triangle is [[Right triangle|right-angled]].
-[[File:Resolve triangle with b c beta 2 solutions.png|thumb|right|250px|<center>Two solutions for triangle</center>]]
+[[File:Resolve triangle with b c beta 2 solutions.png|thumb|right|250px|<center>Two solutions for the triangle</center>]]
-# <li value="3"> If <math>D<1</math> two alternatives are possible.
+# <li value="3"> If {{math|''D'' < 1}} two alternatives are possible.
-## If <math>b<c</math>, the angle <math>\gamma</math> may be acute: <math>~\gamma = \arcsin D</math> or obtuse: <math>~\gamma' = 180^\circ - \gamma</math>. The picture on right shows the point <math>C</math>, the side <math>b</math> and the angle <math>\gamma</math> as the first solution, and the point <math>C'</math>, side <math>b'</math> and the angle <math>\gamma'</math> as the second solution.
+## If {{math|''b'' < ''c''}}, the angle {{math|''γ''}} may be acute: {{math|''γ'' {{=}} arcsin ''D''}} or obtuse: {{math|''γ′'' {{=}} 180° - ''γ''}}. The picture on right shows the point {{math|''C''}}, the side {{math|''b''}} and the angle {{math|''γ''}} as the first solution, and the point {{math|''C′''}}, side {{math|''b′''}} and the angle {{math|''γ′''}} as the second solution.
-## If <math>b \geqslant c</math> then <math>\beta \geqslant \gamma</math> (the larger side corresponds to a larger angle). Since no triangle can have two obtuse angles, <math>~\gamma</math> is acute angle and the solution <math>~\gamma=\arcsin D</math> is unique.
+## If {{math|''b'' ≥ ''c''}} then {{math|''β'' ≥ ''γ''}} (the larger side corresponds to a larger angle). Since no triangle can have two obtuse angles, {{math|''γ''}} is an acute angle and the solution {{math|''γ'' {{=}} arcsin ''D''}} is unique.
-Once <math>\gamma </math> is obtained, the third angle <math>\alpha = 180^\circ - \beta - \gamma </math>.
+Once {{math|''γ''}} is obtained, the third angle {{math|''α'' {{=}} 180° − ''β'' − ''γ''}}.
 The third side can then be found from the law of sines:
 : <math>a = b\ \frac{\sin\alpha}{\sin\beta}</math>
-[[File:resolve triangle with c alpha beta.png|thumb|right|250px|<center>A side and two adjacent angles given</center>]]
+[[File:resolve triangle with c alpha beta.png|thumb|right|250px|<center>One side and two adjacent angles given</center>]]
 === A side and two adjacent angles given (ASA) ===
-The known characteristics are the side  <math>c</math> and the angles <math>\alpha, \beta</math>. The third angle <math>~\gamma = 180^\circ - \alpha - \beta.</math>
+The known characteristics are the side  {{math|''c''}} and the angles {{math|''α'', ''β''}}. The third angle {{math|''γ'' {{=}} 180° − ''α'' − ''β''}}.
 Two unknown side can be calculated from the law of sines:<ref>{{cite web |url=http://www.mathsisfun.com/algebra/trig-solving-asa-triangles.html |title=Solving ASA Triangles |publisher=Maths is Fun |accessdate=13 January 2015}}</ref>
@@ Line 86: / Line 86: @@
 == Solving spherical triangles ==
-The general [[spherical triangle]] is fully determined by three of its six characteristics (3 sides and 3 angles). Note that the sides <math> a, b, c </math> of a spherical triangle are measured by angular rather than linear units, based on the corresponding [[central angle]]s.
+The general [[spherical triangle]] is fully determined by three of its six characteristics (3 sides and 3 angles). Note that the sides {{math|''a'', ''b'', ''c''}} of a spherical triangle are measured by angular rather than linear units, based on the corresponding [[central angle]]s.
-The solution of triangles for [[Non-Euclidean geometry|non-Euclidean]] [[spherical geometry]] has some differences from the planar case. For example, the sum of the three angles <math> \alpha + \beta + \gamma </math> depends on the triangle. In addition, there are no unequal [[Similarity (geometry)|similar triangles]], and so the problem of constructing a triangle with specified three angles has a unique solution. The basic relations used to solve a problem are similar to those of the planar case: see [[Law of cosines (spherical)]] and [[Law of sines#Spherical case|Law of sines (spherical)]].
+The solution of triangles for [[Non-Euclidean geometry|non-Euclidean]] [[spherical geometry]] has some differences from the planar case. For example, the sum of the three angles {{math|''α'' + ''β'' + ''γ''}} depends on the triangle. In addition, there are no unequal [[Similarity (geometry)|similar triangles]], and so the problem of constructing a triangle with specified three angles has a unique solution. The basic relations used to solve a problem are similar to those of the planar case: see [[Law of cosines (spherical)]] and [[Law of sines#Spherical case|Law of sines (spherical)]].
 Among other relationships that may be useful are the [[half-side formula]] and [[Napier's analogies]]:<ref>[http://mathworld.wolfram.com/NapiersAnalogies.html Napier's Analogies] at MathWorld</ref>
@@ Line 99: / Line 99: @@
 === Three sides given (spherical SSS) ===
-Known: the sides <math>a, b, c</math> (in angular units). The triangle's angles are computed from [[law of cosines (spherical)|the spherical law of cosines]]:
+Known: the sides {{math|''a'', ''b'', ''c''}} (in angular units). The triangle's angles are computed from [[law of cosines (spherical)|the spherical law of cosines]]:
 : <math>\alpha = \arccos\left(\frac{\cos a-\cos b\ \cos c}{\sin b\ \sin c}\right),</math>
 : <math>\beta  = \arccos\left(\frac{\cos b-\cos c\ \cos a}{\sin c\ \sin a}\right),</math>
@@ Line 108: / Line 108: @@
 === Two sides and the included angle given (spherical SAS) ===
-Known: the sides <math>a, b</math> and the angle <math>\gamma</math> between them. The side <math>c</math> can be found from the law of cosines:
+Known: the sides {{math|''a'', ''b''}} and the angle {{math|''γ''}} between them. The side {{math|''c''}} can be found from the law of cosines:
 : <math>c = \arccos \left(\cos a\cos b + \sin a\sin b\cos\gamma \right).</math>
-The angles <math>\alpha, \beta</math> can be calculated as above, or by using Napier's analogies:
+The angles {{math|''α', ''β''}} can be calculated as above, or by using Napier's analogies:
 : <math>\alpha = \arctan\ \frac{2\sin a}{\tan(\frac{\gamma}{2}) \sin (b+a) + \cot(\frac{\gamma}{2})\sin(b-a)},</math>
@@ Line 118: / Line 118: @@
 : <math>\beta = \arctan\ \frac{2\sin b}{\tan(\frac{\gamma}{2}) \sin (a+b) + \cot(\frac{\gamma}{2})\sin(a-b) }.</math>
-This problem arises in the [[Great-circle navigation#Course and distance|navigation problem]]
+This problem arises in the [[Great-circle navigation#Course and distance|navigation problem]] of finding the great circle between two points on the earth specified by their latitude and longitude; in this application, it is important to use formulas which are not susceptible to round-off errors.  For this purpose, the following formulas (which may be derived using vector algebra) can be used:
-of finding the great circle between two points on the earth specified by their
-latitude and longitude; in this application, it is important to use formulas which are not
-susceptible to round-off errors.  For this purpose, the following formulas (which may be
-derived using vector algebra) can be used:
 :<math>\begin{align}
 c &= \arctan\frac
@@ Line 134: / Line 130: @@
 {\sin a\cos b - \cos a\sin b\cos\gamma},
 \end{align}</math>
-where the signs of the numerators and denominators in these expressions
+where the signs of the numerators and denominators in these expressions should be used to determine the quadrant of the arctangent.
-should be used to determine the quadrant of the arctangent.
 {{clear}}
-[[File:Solve spherical triangle with b c beta.png|thumb|right|250px|<center>Two sides and non-included angle given</center>]]
+[[File:Solve spherical triangle with b c beta.png|thumb|right|250px|<center>Two sides and a non-included angle given</center>]]
 === Two sides and non-included angle given (spherical SSA) ===
-This problem is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length.  Known: the sides <math>b, c</math> and the angle <math>\beta</math> not between them. A solution exists if the following condition holds:
+This problem is not solvable in all cases; a solution is guaranteed to be unique only if the side length adjacent to the angle is shorter than the other side length.  Known: the sides {{math|''b'', ''c''}} and the angle {{math|''β''}} not between them. A solution exists if the following condition holds:
 : <math>b > \arcsin (\sin c\,\sin\beta).</math>
-The angle <math>\gamma</math> can be found from the [[Law of sines#Spherical case|spherical law of sines]]:
+The angle {{math|''γ''}} can be found from the [[Law of sines#Spherical case|spherical law of sines]]:
 : <math>\gamma = \arcsin \left(\frac{\sin c\,\sin\beta}{\sin b}\right).</math>
-As for the plane case, if <math>b<c</math> then there are two solutions: <math>\gamma</math> and <math>~180^\circ - \gamma.</math>
+As for the plane case, if {{math|''b'' < ''c''}} then there are two solutions: {{math|''γ''}} and {{math|180° - ''γ''}}.
 We can find other characteristics by using Napier's analogies:
-: <math>a = 2\arctan \left\{ \tan\left(\frac12(b-c)\right) \frac{\sin \left(\frac12(\beta+\gamma)\right)}{\sin\left(\frac12(\beta-\gamma)\right)} \right\},</math>
+: <math>a = 2\arctan \left[ \tan\left(\tfrac12(b-c)\right) \frac{\sin \left(\tfrac12(\beta+\gamma)\right)}{\sin\left(\tfrac12(\beta-\gamma)\right)} \right],</math>
-: <math>\alpha = 2\arccot \left\{\tan\left(\frac12(\beta-\gamma)\right) \frac{\sin \left(\frac12(b+c)\right)}{\sin \left(\frac12(b-c)\right)} \right\}.</math>
+: <math>\alpha = 2\arccot \left[\tan\left(\tfrac12(\beta-\gamma)\right) \frac{\sin \left(\tfrac12(b+c)\right)}{\sin \left(\tfrac12(b-c)\right)} \right].</math>
-[[File:Solve spherical triangle with c alpha beta.png|thumb|right|250px|<center>A side and two adjacent angles given</center>]]
+[[File:Solve spherical triangle with c alpha beta.png|thumb|right|250px|<center>One side and two adjacent angles given</center>]]
 === A side and two adjacent angles given (spherical ASA) ===
-Known: the side <math>c</math> and the angles <math>\alpha, \beta</math>. First we determine the angle <math>\gamma</math> using the [[Law of cosines (spherical)|law of cosines]]:
+Known: the side {{math|''c''}} and the angles {{math|''α'', ''β''}}. First we determine the angle {{math|''γ''}} using the [[Law of cosines (spherical)|law of cosines]]:
 : <math>\gamma = \arccos(\sin\alpha\sin\beta\cos c -\cos\alpha\cos\beta).\,</math>
-We can find the two unknown sides from the law of cosines (using the calculated angle <math>\gamma</math>):
+We can find the two unknown sides from the law of cosines (using the calculated angle {{math|''γ''}}):
 : <math>a=\arccos\left(\frac{\cos\alpha+\cos\beta\cos\gamma}{\sin\beta\sin\gamma}\right),</math>
 : <math>b=\arccos\left(\frac{\cos\beta+\cos\gamma\cos\alpha}{\sin\gamma\sin\alpha}\right),</math>
 or by using Napier's analogies:
-: <math>a = \arctan\left\{\frac{2\sin\alpha}{\cot(c/2) \sin(\beta+\alpha) + \tan(c/2) \sin(\beta-\alpha)}\right\},</math>
+: <math>a = \arctan\left[\frac{2\sin\alpha}{\cot(\frac{c}{2}) \sin(\beta+\alpha) + \tan(\frac{c}{2}) \sin(\beta-\alpha)}\right],</math>
-: <math>b = \arctan\left\{\frac{2\sin\beta} {\cot(c/2) \sin(\alpha+\beta) + \tan(c/2)\sin(\alpha-\beta)}\right\}.</math>
+: <math>b = \arctan\left[\frac{2\sin\beta} {\cot(\frac{c}{2}) \sin(\alpha+\beta) + \tan(\frac{c}{2})\sin(\alpha-\beta)}\right].</math>
 {{clear}}
-[[File:Solve spherical triangle with a alpha beta.png|thumb|right|250px|<center>A side, one adjacent angle and the opposite angle given</center>]]
+[[File:Solve spherical triangle with a alpha beta.png|thumb|right|250px|<center>One side, one adjacent angle and the opposite angle given</center>]]
 === A side, one adjacent angle and the opposite angle given (spherical AAS) ===
-Known: the side <math>a</math> and the angles <math>\alpha, \beta</math>. The side <math>b</math> can be found from the [[Law of sines#Spherical case|law of sines]]:
+Known: the side {{math|''a''}} and the angles {{math|''α'', ''β''}}. The side {{math|''b''}} can be found from the [[Law of sines#Spherical case|law of sines]]:
 : <math>b = \arcsin \left( \frac{\sin a\,\sin \beta}{\sin \alpha} \right).</math>
-If the angle for the side <math>a</math> is acute and <math>\alpha > \beta</math>, another solution exists:
+If the angle for the side {{math|''a''}} is acute and {{math|''α'' > ''β''}}, another solution exists:
 : <math>b = \pi - \arcsin \left( \frac{\sin a\,\sin \beta}{\sin \alpha} \right).</math>
 We can find other characteristics by using Napier's analogies:
-: <math>c =  2\arctan \left\{ \tan\left(\frac12(a-b)\right) \frac{\sin\left(\frac12(\alpha+\beta)\right)}{\sin\left(\frac12(\alpha-\beta)\right)}\right\},</math>
+: <math>c =  2\arctan \left[ \tan\left(\tfrac12(a-b)\right) \frac{\sin\left(\tfrac12(\alpha+\beta)\right)}{\sin\left(\frac12(\alpha-\beta)\right)}\right],</math>
-: <math>\gamma = 2\arccot \left\{\tan\left(\frac12(\alpha-\beta)\right) \frac{\sin \left(\frac12(a+b)\right)}{\sin \left(\frac12(a-b)\right)} \right\}.</math>
+: <math>\gamma = 2\arccot \left[\tan\left(\tfrac12(\alpha-\beta)\right) \frac{\sin \left(\tfrac12(a+b)\right)}{\sin \left(\frac12(a-b)\right)} \right].</math>
 {{clear}}
@@ Line 179: / Line 175: @@
 === Three angles given (spherical AAA) ===
-Known: the angles <math>\alpha, \beta, \gamma</math>. From the [[Law of cosines (spherical)|law of cosines]] we infer:
+Known: the angles {{math|''α'', ''β'', ''γ''}}. From the [[Law of cosines (spherical)|law of cosines]] we infer:
 : <math>a=\arccos\left(\frac{\cos\alpha+\cos\beta\cos\gamma}{\sin\beta\sin\gamma}\right),</math>
 : <math>b=\arccos\left(\frac{\cos\beta+\cos\gamma\cos\alpha}{\sin\gamma\sin\alpha}\right),</math>
@@ Line 186: / Line 182: @@
 === Solving right-angled spherical triangles ===
-The above algorithms become much simpler if one of the angles of a triangle (for example, the angle <math>C</math>) is the right angle. Such a spherical triangle is fully defined by its two elements, and the other three can be calculated using [[Spherical trigonometry#Napier's Pentagon|Napier's Pentagon]] or the following relations.
+The above algorithms become much simpler if one of the angles of a triangle (for example, the angle {{math|''C''}}) is the right angle. Such a spherical triangle is fully defined by its two elements, and the other three can be calculated using [[Spherical trigonometry#Napier's Pentagon|Napier's Pentagon]] or the following relations.
 : <math>\sin a = \sin c \cdot \sin A</math> (from the [[Law of sines#Spherical case|Law of sines (spherical)]])
 : <math>\tan a = \sin b \cdot \tan A</math>
@@ Line 199: / Line 195: @@
 [[File:distance by triangulation.svg|thumb|right|320px|<center>Distance measurement by [[triangulation]]</center>]]
   {{main|Triangulation}}
-If one wants to measure the distance <math>d</math>  from shore to a remote ship via triangulation, one marks on the shore two points with known distance <math>l</math> between them (base line). Let <math>\alpha,\beta</math> be the angles between base line and the direction to ship.
+If one wants to measure the distance {{math|''d''}} from shore to a remote ship via triangulation, one marks on the shore two points with known distance {{math|''l''}} between them (the baseline). Let {{math|''α'', ''β''}} be the angles between the baseline and the direction to the ship.
-From the formulas above (ASA case) one can define the length of the [[Triangle|triangle height]]:
+From the formulae above (ASA case) one can define the length of the [[Triangle|triangle height]]:
 : <math>d = \frac{\sin\alpha\,\sin\beta}{\sin(\alpha+\beta)} \,l = \frac{\tan\alpha\,\tan\beta}{\tan\alpha+\tan\beta} \,l.</math>
-This method is used in [[cabotage]]. The angles <math>\alpha, \beta</math> are defined  by observations familiar landmarks from the ship.
+This method is used in [[cabotage]]. The angles {{math|''α'', ''β''}} are defined  by observation of familiar landmarks from the ship.
 {{clear}}
-[[File:mountain height by triangulation.png|thumb|right|320px|<center>How to measure a mountain height</center>]]
+[[File:mountain height by triangulation.png|thumb|right|320px|<center>How to measure a mountain's height</center>]]
-As another example, if one wants to measure the height <math>h</math> of a mountain or a high building, the angles <math>\alpha, \beta</math> from two ground points to the top are specified. Let <math>l</math> be the distance between tis points. From the same ASA case formulas we obtain:
+As another example, if one wants to measure the height {{math|''h''}} of a mountain or a high building, the angles {{math|''α'', ''β''}} from two ground points to the top are specified. Let {{math|''l''}} be the distance between these points. From the same ASA case formulas we obtain:
 : <math> h = \frac{\sin\alpha\,\sin\beta}{\sin(\beta-\alpha)} \,l = \frac{\tan\alpha\,\tan\beta}{\tan\beta-\tan\alpha} \,l.</math>
@@ Line 212: / Line 208: @@
 [[File:distance on earth.png|224px|right]]
 To calculate the distance between two points on the globe,
-: Point A: latitude  <math>\lambda_\mathrm{A},</math> longitude  <math>L_\mathrm{A},</math> and
+: Point A: latitude {{math|''λ{{sub|A}}''}}, longitude {{math|''L{{sub|A}}''}}, and
-: Point B: latitude <math>\lambda_\mathrm{B},</math> longitude  <math>L_\mathrm{B},</math>
+: Point B: latitude {{math|''λ{{sub|B}}''}}, longitude {{math|''L{{sub|B}}''}}
-we consider the spherical triangle<math>ABC</math>, where <math>C</math> is the North Pole. Some characteristics are:
+we consider the spherical triangle {{math|''ABC''}}, where {{math|''C''}} is the North Pole. Some characteristics are:
 : <math>a = 90^\mathrm{o} - \lambda_\mathrm{B}, \,</math>
 : <math>b = 90^\mathrm{o} - \lambda_\mathrm{A}, \,</math>
 : <math>\gamma = L_\mathrm{A}-L_\mathrm{B}. \,</math>
 If [[#Two sides and the included angle given|two sides and the included angle given]], we obtain from the formulas
-: <math>\mathrm{AB} = R \arccos\left\{\sin \lambda_\mathrm{A} \,\sin \lambda_\mathrm{B} + \cos \lambda_\mathrm{A} \,\cos \lambda_\mathrm{B} \,\cos \left(L_\mathrm{A}-L_\mathrm{B}\right)\right\}.</math>
+: <math>\mathrm{AB} = R \arccos\left[\sin \lambda_\mathrm{A} \,\sin \lambda_\mathrm{B} + \cos \lambda_\mathrm{A} \,\cos \lambda_\mathrm{B} \,\cos \left(L_\mathrm{A}-L_\mathrm{B}\right)\right].</math>
-Here <math>R</math> is the [[Earth|Earth radius]].
+Here {{math|''R''}} is the [[Earth|Earth's radius]].
 == See also ==