View source for Concyclic points

{{Short description|Points on a common circle}}
[[File:Concyclic.svg|thumb|[[Concurrent lines|Concurrent]] perpendicular bisectors of [[Circle#Chord|chords]] between concyclic points]]
[[File:Circumscribed Polygon.svg|thumb|Circumscribed circle, {{mvar|C}}, and circumcenter, {{mvar|O}}, of a ''cyclic polygon'', {{mvar|P}}]]

In [[geometry]], a [[set (mathematics)|set]] of [[point (geometry)|points]] are said to be '''concyclic''' (or '''cocyclic''') if they lie on a common [[circle]]. A [[polygon]] whose [[vertex (geometry)|vertices]] are concyclic is called a '''cyclic polygon''', and the circle is called its ''circumscribing circle'' or ''circumcircle''. All concyclic points are [[equidistant]] from the center of the circle.

Three points in the [[Euclidean plane|plane]] that do not all fall on a [[straight line]] are concyclic, so every [[triangle]] is a cyclic polygon, with a well-defined [[circumcircle]]. However, four or more points in the plane are not necessarily concyclic. After triangles, the special case of [[cyclic quadrilateral]]s has been most extensively studied.

== Perpendicular bisectors ==
In general the centre ''O'' of a circle on which points ''P'' and ''Q'' lie must be such that ''OP'' and ''OQ'' are equal distances. Therefore ''O'' must lie on the [[perpendicular bisector]] of the line segment ''PQ''.<ref>{{citation
 | last = Libeskind | first = Shlomo
 | isbn = 9780763743666
 | page = 21
 | publisher = Jones & Bartlett Learning
 | title = Euclidean and Transformational Geometry: A Deductive Inquiry
 | url = https://books.google.com/books?id=6YUUeO-RjU0C&pg=PA21
 | year = 2008}}/</ref> For ''n'' distinct points there are [[triangular number|''n''(''n''&nbsp;−&nbsp;1)/2]] bisectors, and the concyclic condition is that they all meet in a single point, the centre ''O''.

== Triangles ==
{{main|Circumcircle}}

The vertices of every [[triangle]] fall on a circle called the [[circumcircle]]. (Because of this, some authors define "concyclic" only in the context of four or more points on a circle.)<ref>{{citation
 | last = Elliott | first = John
 | page = 126
 | publisher = Swan Sonnenschein & co.
 | title = Elementary Geometry
 | url = https://books.google.com/books?id=9psBAAAAYAAJ&pg=PA126
 | year = 1902}}.</ref> Several other sets of points defined from a triangle are also concyclic, with different circles; see [[Nine-point circle]]<ref>{{citation
 | last = Isaacs | first = I. Martin
 | author-link=Martin Isaacs
 | isbn = 9780821847947
 | page = 63
 | publisher = American Mathematical Society
 | series = Pure and Applied Undergraduate Texts
 | title = Geometry for College Students
 | url = https://books.google.com/books?id=0ahK8UneO3kC&pg=PA63
 | volume = 8
 | year = 2009}}.</ref> and [[Lester's theorem]].<ref>{{citation
 | last = Yiu | first = Paul
 | journal = Forum Geometricorum
 | mr = 2868943
 | pages = 175–209
 | title = The circles of Lester, Evans, Parry, and their generalizations
 | url = http://forumgeom.fau.edu/FG2010volume10/FG201020.pdf
 | volume = 10
 | year = 2010}}.</ref>

The [[radius]] of the circle on which lie a set of points is, by definition, the radius of the circumcircle of any triangle with vertices at any three of those points. If the pairwise distances among three of the points are ''a'', ''b'', and ''c'', then the circle's radius is

:<math>R = \sqrt{\frac{a^2b^2c^2}{(a+b+c)(-a+b+c)(a-b+c)(a+b-c)}}.</math>

The equation of the circumcircle of a triangle, and expressions for the radius and the coordinates of the circle's center, in terms of the Cartesian coordinates of the vertices are given [[Circumcircle#Circumcircle equations|here]].

=== Other concyclic points ===

In any triangle all of the following nine points are concyclic on what is called the [[nine-point circle]]: the midpoints of the three edges, the feet of the three [[altitude (geometry)|altitudes]], and the points halfway between the [[orthocenter]] and each of the three vertices.

[[Lester's theorem]] states that in any [[scalene triangle]], the two [[Fermat point]]s, the [[nine-point center]], and the [[circumcenter]] are concyclic.

If [[Line (mathematics)|lines]] are drawn through the [[Lemoine point]] [[parallel (geometry)|parallel]] to the sides of a triangle, then the six points of intersection of the lines and the sides of the triangle are concyclic, in what is called the [[Lemoine circle]].

The [[van Lamoen circle]] associated with any given triangle <math>T</math> contains the [[circumcenter]]s of the six triangles that are defined inside <math>T</math> by its three [[median (geometry)|median]]s.

A triangle's [[circumcenter]], its [[Lemoine point]], and its first two [[Brocard points]] are concyclic, with the segment from the circumcenter to the Lemoine point being a [[diameter]].<ref>Scott, J. A. "Some examples of the use of areal coordinates in triangle geometry", ''[[Mathematical Gazette]]'' 83, November 1999, 472–477.</ref>

== Cyclic quadrilaterals ==
{{main|Cyclic quadrilateral}}

[[File:Four concyclic points.png|thumb|Four concyclic points forming a [[cyclic quadrilateral]], showing two equal angles]]

A quadrilateral ''ABCD'' with concyclic vertices is called a [[cyclic quadrilateral]]; this happens [[if and only if]] <math>\angle CAD = \angle CBD</math> (the  [[inscribed angle theorem]]) which is true if and only if the opposite angles inside the quadrilateral are [[supplementary angle|supplementary]].<ref>{{citation
 | last = Pedoe | first = Dan
 | edition = 2nd
 | isbn = 9780883855188
 | page = xxii
 | publisher = Cambridge University Press
 | series = MAA Spectrum
 | title = Circles: A Mathematical View
 | url = https://books.google.com/books?id=rlbQTxbutA4C&pg=PR22
 | year = 1997}}.</ref> A cyclic quadrilateral with successive sides ''a'', ''b'', ''c'', ''d'' and [[semiperimeter]] ''s'' = (''a'' + ''b'' + ''c'' + ''d'')&thinsp;/&thinsp;2 has its circumradius given by<ref name=Alsina2>{{citation
|last1=Alsina |first1=Claudi |last2=Nelsen |first2=Roger B.
|journal=Forum Geometricorum
|pages=147–9
|title=On the diagonals of a cyclic quadrilateral
|url=http://forumgeom.fau.edu/FG2007volume7/FG200720.pdf |volume=7
|year=2007}}</ref><ref>{{citation |last=Hoehn |first=Larry |title=Circumradius of a cyclic quadrilateral |journal=Mathematical Gazette |volume=84 |issue=499 |date=March 2000 |pages=69–70  |doi=10.2307/3621477 |jstor=3621477}}</ref>
:<math>R=\frac{1}{4} \sqrt{\frac{(ab+cd)(ac+bd)(ad+bc)}{(s-a)(s-b)(s-c)(s-d)}},</math>
an expression that was derived by the Indian mathematician Vatasseri [[Parameshvara]] in the 15th century.

By [[Ptolemy's theorem]], if a quadrilateral is given by the pairwise distances between its four vertices ''A'', ''B'', ''C'', and ''D'' in order, then it is cyclic if and only if the product of the diagonals equals the sum of the products of opposite sides: 

: <math>AC \cdot BD = AB \cdot  CD + BC \cdot AD.</math>

If two lines, one  containing segment ''AC'' and the other containing segment ''BD'', intersect at ''X'', then the four points ''A'', ''B'', ''C'', ''D'' are concyclic if and only if<ref>{{citation |last=Bradley |first=Christopher J. |title=The Algebra of Geometry: Cartesian, Areal and Projective Co-Ordinates |publisher=Highperception |year=2007 |isbn=978-1906338008 |page=179 |oclc=213434422}}</ref>
:<math>\displaystyle AX\cdot XC = BX\cdot XD.</math>

The intersection ''X'' may be internal or external to the circle. This theorem is known as [[power of a point]].

A convex quadrilateral is [[orthodiagonal]] (has perpendicular diagonals) if and only if the midpoints of the sides and the feet of the four [[Quadrilateral#Special line segments|altitudes]] are eight concyclic points, on what is called the '''eight-point circle'''.

== Cyclic polygons ==
[[File:annuli_with_same_area_around_unit_regular_polygons.svg|thumb|As a corollary of the [[annulus (mathematics)|annulus]] chord formula, the&nbsp;area bounded by the [[circumcircle]] and [[incircle]] of every unit regular {{mvar|n}}-gon is {{pi}}/4]]

More generally, a [[polygon]] in which all vertices are concyclic is called a ''cyclic polygon''. A polygon is cyclic if and only if the perpendicular bisectors of its edges are [[concurrent lines|concurrent]].<ref>{{citation
 | last1 = Byer | first1 = Owen
 | last2 = Lazebnik | first2 = Felix
 | last3 = Smeltzer | first3 = Deirdre L. | author3-link = Deirdre Smeltzer
 | isbn = 9780883857632
 | page = 77
 | publisher = Mathematical Association of America
 | title = Methods for Euclidean Geometry
 | url = https://books.google.com/books?id=W4acIu4qZvoC&pg=PA77
 | year = 2010}}.</ref> Every [[regular polygon]] is a cyclic polygon.

For a cyclic polygon with an odd number of sides, all angles are equal if and only if the polygon is regular. A cyclic polygon with an even number of sides has all angles equal if and only if the alternate sides are equal (that is, sides {{nowrap|1, 3, 5, …}} are equal, and sides {{nowrap|2, 4, 6, …}} are equal).<ref>{{cite journal|last=De Villiers|first=Michael|title=95.14 Equiangular cyclic and equilateral circumscribed polygons|journal=[[The Mathematical Gazette]]|volume=95|issue= 532 |date=March 2011|pages=102–107|doi=10.1017/S0025557200002461|jstor= 23248632|s2cid=233361080 }}</ref>

A cyclic [[pentagon]] with [[rational number|rational]] sides and area is known as a [[Robbins pentagon]]. In all known cases, its diagonals also have rational lengths, though whether this is true for all possible Robbins pentagons is an unsolved problem.<ref>{{cite journal|last1=Buchholz|first1=Ralph H.|last2=MacDougall|first2=James A.|doi=10.1016/j.jnt.2007.05.005|issue=1|journal=[[Journal of Number Theory]]|mr=2382768|pages=17–48|title=Cyclic polygons with rational sides and area|volume=128|year=2008|doi-access=free}}</ref>

In any cyclic {{mvar|n}}-gon with even {{mvar|n}}, the sum of one set of alternate angles (the first, third, fifth, etc.) equals the sum of the other set of alternate angles. This can be proven by induction from the {{math|1=''n'' = 4}} case, in each case replacing a side with three more sides and noting that these three new sides together with the old side form a quadrilateral which itself has this property; the alternate angles of the latter quadrilateral represent the additions to the alternate angle sums of the previous {{mvar|n}}-gon.

A [[tangential polygon]] is one having an [[inscribed circle]] tangent to each side of the polygon; these tangency points are thus concyclic on the inscribed circle. Let one {{mvar|n}}-gon be inscribed in a circle, and let another {{mvar|n}}-gon be tangential to that circle at the vertices of the first {{mvar|n}}-gon. Then from any point {{mvar|P}} on the circle, the product of the perpendicular distances from {{mvar|P}} to the sides of the first {{mvar|n}}-gon equals the product of the perpendicular distances from {{mvar|P}} to the sides of the second {{mvar|n}}-gon.<ref>{{cite book |first=Roger A. |last=Johnson |title=Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle |publisher=Houghton Mifflin Co. |year=1929 |hdl=2027/wu.89043163211 |page=72}} Republished by Dover Publications as ''Advanced Euclidean Geometry'', 1960 and 2007.</ref>

===Point on the circumcircle===

Let a cyclic {{mvar|n}}-gon have vertices {{math|''A''{{sub|1}} , …, ''A{{sub|n}}''}} on the unit circle. Then for any point {{mvar|M}} on the minor arc {{math|''A''{{sub|1}}''A{{sub|n}}''}}, the distances from {{mvar|M}} to the vertices satisfy<ref>{{cite web|title=Inequalities proposed in ''Crux Mathematicorum''|work=The IMO Compendium|url=http://www.imomath.com/othercomp/Journ/ineq.pdf|at=p. 190, #332.10}}</ref>

:<math>\begin{cases}
  \overline{MA_1} + \overline{MA_3} + \cdots + \overline{MA_{n-2}} + \overline{MA_n} < n/\sqrt{2} & \text{if } n \text{ is odd}; \\
  \overline{MA_1} + \overline{MA_3} + \cdots + \overline{MA_{n-3}} + \overline{MA_{n-1}} \leq n/\sqrt{2} & \text{if } n \text{ is even}.
\end{cases}</math>

For a regular {{mvar|n}}-gon, if <math>\overline{MA_i}</math> are the distances from any point {{mvar|M}} on the circumcircle to the vertices {{mvar|A{{sub|i}}}}, then <ref name=Mamuka >{{cite journal| last1= Meskhishvili |first1= Mamuka| date=2020|title=Cyclic Averages of Regular Polygons and Platonic Solids |journal= Communications in Mathematics and Applications|volume=11|pages=335–355|doi= 10.26713/cma.v11i3.1420|doi-broken-date= 1 November 2024|arxiv= 2010.12340|url= https://www.rgnpublications.com/journals/index.php/cma/article/view/1420/1065}}</ref>

:<math>3(\overline{MA_1}^2 + \overline{MA_2}^2 + \dots +  \overline{MA_n}^2)^2=2n (\overline{MA_1}^4 + \overline{MA_2}^4 + \dots + \overline{MA_n}^4).</math>

===Polygon circumscribing constant===
[[File:Kepler constant inverse.svg|thumb|right|upright=0.8|A sequence of circumscribed polygons and circles.]]

Any [[regular polygon]] is cyclic. Consider a unit circle, then circumscribe a regular triangle such that each side touches the circle. Circumscribe a circle, then circumscribe a square. Again circumscribe a circle, then circumscribe a regular [[pentagon]], and so on. The radii of the circumscribed circles converge to the so-called ''polygon circumscribing constant''

:<math>\prod_{n=3}^\infty \frac 1 {\cos\left( \frac\pi n \right)} = 8.7000366\ldots.</math>

{{OEIS|A051762}}. The reciprocal of this constant is the [[Kepler–Bouwkamp constant]].

==Variations==

In contexts where lines are taken to be a type of [[generalised circle]] with infinite radius, [[collinear points]] (points along a single line) are considered to be concyclic. This point of view is helpful, for instance, when studying [[circle inversion|inversion through a circle]] or more generally [[Möbius transformation]]s (geometric transformations generated by reflections and circle inversions), as these transformations preserve the concyclicity of points only in this extended sense.<ref>{{citation
 | last = Zwikker | first = C.
 | authorlink = Cornelis Zwikker
 | isbn = 9780486442761
 | page = 24
 | publisher = Courier Dover Publications
 | title = The Advanced Geometry of Plane Curves and Their Applications
 | url = https://books.google.com/books?id=25tMEYTik-AC&pg=PA24
 | year = 2005}}.</ref>

In the [[complex plane]] (formed by viewing the [[real and imaginary parts]] of a [[complex number]] as the ''x'' and ''y'' [[Cartesian coordinates]] of the plane), concyclicity has a particularly simple formulation: four points in the complex plane are either concyclic or collinear if and only if their [[cross-ratio]] is a [[real number]].<ref>{{citation
 | last = Hahn | first = Liang-shin
 | edition = 2nd
 | isbn = 9780883855102
 | page = 65
 | publisher = Cambridge University Press
 | series = MAA Spectrum
 | title = Complex Numbers and Geometry
 | url = https://books.google.com/books?id=s3nMMkPEvqoC&pg=PA65
 | year = 1996}}.</ref>

==Integer area and side lengths==
Some cyclic polygons have the property that their area and all of their side lengths are positive integers.  Triangles with this property are called [[Heronian triangle]]s; cyclic quadrilaterals with this property (and that the diagonals that connect opposite vertices have integer length) are called [[Brahmagupta quadrilateral]]s; cyclic pentagons with this property are called [[Robbins pentagon]]s.  More generally, versions of these cyclic polygons scaled by a [[rational number]] will have area and side lengths that are rational numbers.

Let {{math|''θ''<sub>1</sub>}} be the angle spanned by one side of the cyclic polygon as viewed from the center of the circumscribing circle.  Similarly define the [[central angle]]s {{math|''θ''<sub>2</sub>, ..., ''θ''<sub>''n''</sub>}} for the remaining {{math|''n'' − 1}} sides.  Every Heronian triangle and every Brahmagupta quadrilateral has a rational value for the tangent of the quarter angle, {{math|tan ''θ''<sub>''k''</sub>/4}}, for every value of {{mvar|k}}.  Every known Robbins pentagon (has diagonals that have rational length and) has this property, though it is an unsolved problem whether every possible Robbins pentagon has this property.

The reverse is true for all cyclic polygons with any number of sides; if all such central angles have rational tangents for their quarter angles then the implied cyclic polygon circumscribed by the unit circle will simultaneously have rational side lengths and rational area.  Additionally, each diagonal that connects two vertices, whether or not the two vertices are adjacent, will have a rational length.  Such a cyclic polygon can be scaled so that its area and lengths are all integers.

This reverse relationship gives a way to generate cyclic polygons with integer area, sides, and diagonals.  For a polygon with {{mvar|n}} sides, let {{math|0 < ''c''<sub>1</sub> < ... < ''c''<sub>''n''−1</sub> < +∞}} be rational numbers.  These are the tangents of one quarter of the cumulative angles {{math|''θ''<sub>1</sub>}}, {{math|''θ''<sub>1</sub> + ''θ''<sub>2</sub>}}, ..., {{math|''θ''<sub>1</sub> + ... + ''θ''<sub>''n''−1</sub>}}.  Let {{math|1=''q''<sub>1</sub> = ''c''<sub>1</sub>}}, let {{math|1=''q''<sub>''n''</sub> = 1 / ''c''<sub>''n''−1</sub>}}, and let {{math|1=''q''{{sub|''k''}} = (''c''{{sub|''k''}} − ''c''{{sub|''k''−1}}) / (1 + ''c''{{sub|''k''}}''c''{{sub|''k''−1}})}} for {{math|1=''k'' = 2, ..., ''n''−1}}.  These rational numbers are the tangents of the individual quarter angles, using the formula for the tangent of the difference of angles.  Rational side lengths for the polygon circumscribed by the unit circle are thus obtained as {{math|1=''s''<sub>''k''</sub> = 4''q''<sub>''k''</sub> / (1 + ''q''<sub>''k''</sub><sup>2</sup>)}}.  The rational area is {{math|1=''A'' = ∑{{sub|''k''}} 2''q''{{sub|''k''}}(1 − ''q''{{sub|''k''}}{{sup|2}}) / (1 + ''q''{{sub|''k''}}{{sup|2}}){{sup|2}}}}.  These can be made into integers by scaling the side lengths by a shared constant.

==Other properties==
A set of five or more points is concyclic if and only if every four-point [[subset]] is concyclic.<ref>{{citation
 | last = Pedoe | first = Dan
 | isbn = 9780486658124
 | page = 431
 | publisher = Courier Dover Publications
 | title = Geometry: A Comprehensive Course
 | url = https://books.google.com/books?id=s7DDxuoNr_0C&pg=PA431
 | year = 1988}}.</ref> This property can be thought of as an analogue for concyclicity of the [[Helly property]] of convex sets.

== Minimum bounding circle ==

A related notion is the one of a [[Smallest circle problem|minimum bounding circle]], which is the smallest circle that completely contains a set of points. Every set of points in the plane has a unique minimum bounding circle, which may be constructed by a [[linear time]] algorithm.<ref>{{cite journal|first=N.|last=Megiddo|title=Linear-time algorithms for linear programming in '''R'''{{sup|3}} and related problems|journal=SIAM Journal on Computing|volume=12|issue=4|pages=759–776|year=1983|doi=10.1137/0212052|s2cid=14467740}}</ref>

Even if a set of points are concyclic, their circumscribing circle may be different from their minimum bounding circle. For example, for an [[obtuse triangle]], the minimum bounding circle has the longest side as diameter and does not pass through the opposite vertex.

==References==
{{reflist}}

== External links ==
* {{MathWorld |title=Concyclic |urlname=Concyclic}}
*''[http://demonstrations.wolfram.com/FourConcyclicPoints/ Four Concyclic Points]'' by Michael Schreiber, [[The Wolfram Demonstrations Project]].

{{Commonscat|Concyclic points}}

[[Category:Elementary geometry]]
[[Category:Incidence geometry]]