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{{About|the shape and mathematical concept}}
{{About|the shape and mathematical concept}}
{{Infobox polygon
|name = Circle
|image = Circle-withsegments.svg
|caption = A circle with circumference (C) in black, diameter (D) in cyan, radius (R) in red, and centre (O) in magenta.
}}


A '''circle''' is circular in form.
A '''circle''' is a simple [[shape]] in [[Euclidean geometry]]. It is the set of all [[point (geometry)|points]] in a [[plane (mathematics)|plane]] that are at a given distance from a given point, the<!-- This article is written using British English – see WP:ENGVAR, Wikipedia articles should remain with the original variant 'center' is the American English spelling --> [[Centre (geometry)|centre]];<!--see comment before about 'center'--> equivalently it is the curve traced out by a point that moves so that its distance from a given point is constant. The distance between any of the points and the centre is called the [[radius]].

A circle is a simple closed [[curve]] which divides the plane into two regions: an [[interior (topology)|interior]] and an exterior. In everyday use, the term "circle" may be used interchangeably to refer to either the boundary of the figure, or to the whole figure including its interior; in strict technical usage, the circle is the former and the latter is called a [[Disk (mathematics)|disk]].

A circle may also be defined as a special [[ellipse]] in which the two [[Focus (geometry)|foci]] are coincident and the [[eccentricity (mathematics)|eccentricity]] is 0, or the two-dimensional shape enclosing the most area per unit perimeter squared, using [[calculus of variations]].
{{TOC limit|3}}{{-}}<!--To avoid layout breaking when TOC is hidden-->

{{quotation|A circle is a plane figure bounded by one line, and such that all right lines drawn from a certain point within it to the bounding line, are equal. The bounding line is called its circumference and the point, its centre.|Euclid. ''Elements'' Book I. <ref>{{OL|7227282M}} </ref>}}

==Terminology==
*[[Arc (geometry)|Arc]]: any [[Connected space|connected]] part of the circle.
*[[Centre (geometry)|Centre]]: the point equidistant from the points on the circle.
*[[Chord (geometry)|Chord]]: a line segment whose endpoints lie on the circle.
*[[Circumference]]: the length of one circuit along the circle, or the distance around the circle.
*[[Diameter]]: a line segment whose endpoints lie on the circle and which passes through the centre; or the length of such a line segment, which is the largest distance between any two points on the circle. It is a special case of a chord, namely the longest chord, and it is twice the radius.
*Passant: a [[coplanar]] straight line that does not touch the circle.
*[[Radius]]: a line segment joining the centre of the circle to any point on the circle itself; or the length of such a segment, which is half a diameter.
*[[Circular sector|Sector]]: a region bounded by two radii and an arc lying between the radii.
*[[Circular segment|Segment]]: a region, not containing the centre, bounded by a chord and an arc lying between the chord's endpoints.
*[[Secant line|Secant]]: an extended chord, a coplanar straight line cutting the circle at two points.
*[[Semicircle]]: an arc that extends from one of a diameter's endpoints to the other. In non-technical common usage it may mean the diameter, arc, and its interior, a two dimensional region, that is technically called a half-[[Disk (mathematics)|disk]]. A half-disk is a special case of a [[Circular segment|segment]], namely the largest one.
*[[Tangent]]: a coplanar straight line that touches the circle at a single point.
{{-}}
{| style="float:left;" cellspacing="0" cellpadding="0"
|-
|[[Image:CIRCLE LINES.svg|right|thumb|Chord, secant, tangent, radius, and diameter]]
|[[Image:Circle slices.svg|right|thumb|Arc, sector, and segment]]
|}
{{-}}

==History==
[[Image:God the Geometer.jpg|thumb|right|200px|
The [[compass (drafting)|compass]] in this 13th-century manuscript is a symbol of God's act of [[Creation myth|Creation]]. Notice also the circular shape of the [[Halo (religious iconography)|halo]]]]
The word "circle" derives from the [[Greek language|Greek]] ''κίρκος/κύκλος'' (''kirkos/kuklos''), itself a [[metathesis (linguistics)|metathesis]] of the [[Homeric Greek]] ''κρίκος'' (''krikos''), meaning "hoop" or "ring".<ref>[http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkri%2Fkos krikos], Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> The origins of the words "[[circus]]" and "[[wikt:circuit|circuit]]" are closely related.
[[Image:IlkhanateSilkCircular.jpg|left|thumb|200px|Circular piece of silk with Mongol images]][[Image:Shatir500.jpg|right|thumb|200px|Circles in an old [[Arabic]] [[astronomical]] drawing.]]The circle has been known since before the beginning of recorded history. Natural circles would have been observed, such as the Moon, Sun, and a short plant stalk blowing in the wind on sand, which forms a circle shape in the sand. The circle is the basis for the [[wheel]], which, with related inventions such as [[gear]]s, makes much of modern machinery possible. In mathematics, the study of the circle has helped inspire the development of geometry, [[astronomy]], and calculus.

Early [[science]], particularly [[geometry]] and [[astrology and astronomy]], was connected to the divine for most [[History of science in the Middle Ages|medieval scholars]], and many believed that there was something intrinsically "divine" or "perfect" that could be found in circles.<ref>[[Arthur Koestler]], ''[[The Sleepwalkers]]: A History of Man's Changing Vision of the Universe'' (1959)</ref><ref>[[Proclus]], [http://books.google.com/books?id=E1HYAAAAMAAJ ''The Six Books of Proclus, the Platonic Successor, on the Theology of Plato''] Tr. Thomas Taylor (1816) Vol.2, Ch.2, "Of Plato"</ref>

Some highlights in the history of the circle are:
*1700 BCE – The [[Rhind papyrus]] gives a method to find the area of a circular field. The result corresponds to {{frac|256|81}} (3.16049...) as an approximate value of [[Pi|{{pi}}]].<ref>[http://www-history.mcs.st-andrews.ac.uk/history/Chronology/30000BC_500BC.html#1700BC Chronology for 30000 BC to 500 BC]. History.mcs.st-andrews.ac.uk. Retrieved on 2012-05-03.</ref>
[[Image:Toghrol Tower looking up.jpg|left|thumb|200px|[[Tughrul Tower]] from inside]]
*300 BCE – Book 3 of [[Euclid's Elements]] deals with the properties of circles.
*In [[Plato]]'s [[Seventh Letter]] there is a detailed definition and explanation of the circle. Plato explains the perfect circle, and how it is different from any drawing, words, definition or explanation.
*1880 CE– [[Ferdinand von Lindemann|Lindemann]] proves that {{pi}} is [[transcendental number|transcendental]], effectively settling the millennia-old problem of [[squaring the circle]].<ref>[http://www-history.mcs.st-andrews.ac.uk/history/HistTopics/Squaring_the_circle.html Squaring the circle]. History.mcs.st-andrews.ac.uk. Retrieved on 2012-05-03.</ref>{{-}}

==Analytic results==

===Length of circumference===
{{further|Circumference}}
The ratio of a circle's [[circumference]] to its [[diameter]] is [[Pi|{{pi}}]] (pi), an [[Irrational number|irrational]] [[mathematical constant|constant]] approximately equal to 3.141592654. Thus the length of the circumference ''C'' is related to the radius ''r'' and diameter ''d'' by:
:<math>C = 2\pi r = \pi d.\,</math>

===Area enclosed===
[[Image:Circle Area.svg|thumb|Area enclosed by a circle = {{pi}} × area of the shaded square]]
{{Main|Area of a disk}}
As proved by Archimedes, the [[Area of a disk|area enclosed by a circle]] is equal to that of a triangle whose base has the length of the circle's circumference and whose height equals the circle's radius,<ref>''[[Measurement of a Circle]]'' by [[Archimedes]]</ref> which comes to {{pi}} multiplied by the radius squared:
:<math>\mathrm{Area} = \pi r^2.\,</math>

Equivalently, denoting diameter by ''d'',
:<math>\mathrm{Area} = \frac{\pi d^2}{4} \approx 0{.}7854d^2,</math>

that is, approximately 79 percent of the [[Circumscribe|circumscribing]] square (whose side is of length ''d'').

The circle is the plane curve enclosing the maximum area for a given arc length. This relates the circle to a problem in the [[calculus of variations]], namely the [[isoperimetric inequality]].

===Equations===

====Cartesian coordinates====
[[Image:Circle center a b radius r.svg|thumb|right|Circle of radius ''r''&nbsp;=&nbsp;1, centre (''a'', ''b'') =&nbsp;(1.2,&nbsp;−0.5)]]
In an ''x''–''y'' [[Cartesian coordinate system]], the circle with centre [[Coordinate system|coordinates]] (''a'', ''b'') and radius ''r'' is the set of all points (''x'', ''y'') such that
:<math>\left(x - a \right)^2 + \left( y - b \right)^2=r^2.</math>

This [[equation]], known as the Equation of the Circle, follows from the [[Pythagorean theorem]] applied to any point on the circle: as shown in the diagram to the right, the radius is the hypotenuse of a right-angled triangle whose other sides are of length |''x'' − ''a''| and |''y'' − ''b''|. If the circle is centred at the origin (0, 0), then the equation simplifies to
:<math>x^2 + y^2 = r^2.\!\ </math>

The equation can be written in [[parametric equation|parametric form]] using the [[trigonometric function]]s sine and cosine as
:<math>x = a+r\,\cos t,\,</math>
:<math>y = b+r\,\sin t\,</math>
where ''t'' is a [[parametric variable]] in the range 0 to 2{{pi}}, interpreted geometrically as the [[angle]] that the ray from (''a'',&nbsp;''b'') to (''x'',&nbsp;''y'') makes with the positive ''x''-axis.

An alternative parametrisation of the circle is:
:<math>x = a + r \frac{2t}{1+t^2}.\,</math>
:<math>y = b + r \frac{1-t^2}{1+t^2}\,</math>

In this parametrisation, the ratio of ''t'' to ''r'' can be interpreted geometrically as the [[stereographic projection]] of the line passing through the centre parallel to the ''x''-axis (see [[Tangent half-angle substitution]]). However, this parametrisation works only if t is made to range not only through all reals but also to a point at infinity; otherwise, the bottom-most point of the circle would be omitted.

In [[homogeneous coordinates]] each [[conic section]] with the equation of a circle has the form
:<math>x^2+y^2-2axz-2byz+cz^2 = 0.\,</math>

It can be proven that a conic section is a circle exactly when it contains (when extended to the [[complex projective plane]]) the points ''I''(1: ''i'': 0) and ''J''(1:&nbsp;−''i'':&nbsp;0). These points are called the [[circular points at infinity]].

====Polar coordinates====
In [[polar coordinates]] the equation of a circle is:
:<math>r^2 - 2 r r_0 \cos(\theta - \phi) + r_0^2 = a^2\,</math>

where ''a'' is the radius of the circle, <math>(r, \theta)</math> is the polar coordinate of a generic point on the circle, and <math>(r_0, \phi)</math> is the polar coordinate of the centre of the circle (i.e., ''r''<sub>0</sub> is the distance from the origin to the centre of the circle, and ''φ'' is the anticlockwise angle from the positive ''x''-axis to the line connecting the origin to the centre of the circle). For a circle centred at the origin, i.e. ''r''<sub>0</sub> = 0, this reduces to simply {{nowrap|''r'' {{=}} ''a''}}. When {{nowrap|''r''<sub>0</sub> {{=}} ''a''}}, or when the origin lies on the circle, the equation becomes
:<math>r = 2 a\cos(\theta - \phi).\,</math>

In the general case, the equation can be solved for ''r'', giving
:<math>r = r_0 \cos(\theta - \phi) \pm \sqrt{a^2 - r_0^2 \sin^2(\theta - \phi)},</math>
Note that without the ± sign, the equation would in some cases describe only half a circle.

====Complex plane====
In the [[complex plane]], a circle with a centre at ''c'' and radius (''r'') has the equation <math>|z-c| = r\,</math>. In parametric form this can be written <math>z = re^{it}+c</math>.

The slightly generalised equation <math>pz\overline{z} + gz + \overline{gz} = q</math> for real ''p'', ''q'' and complex ''g'' is sometimes called a [[generalised circle]]. This becomes the above equation for a circle with <math>p = 1,\ g=-\overline{c},\ q=r^2-|c|^2</math>, since <math>|z-c|^2 = z\overline{z}-\overline{c}z-c\overline{z}+c\overline{c}</math>. Not all generalised circles are actually circles: a generalised circle is either a (true) circle or a [[line (geometry)|line]].

===Tangent lines===
{{Main|Tangent lines to circles}}
The [[tangent line]] through a point ''P'' on the circle is perpendicular to the diameter passing through ''P''. If {{nowrap|P {{=}} (''x''<sub>1</sub>, ''y''<sub>1</sub>)}} and the circle has centre (''a'', ''b'') and radius ''r'', then the tangent line is perpendicular to the line from (''a'', ''b'') to (''x''<sub>1</sub>, ''y''<sub>1</sub>), so it has the form {{nowrap|(''x''<sub>1</sub> − ''a'')''x'' + (''y''<sub>1</sub> – ''b'')''y'' {{=}} ''c''}}. Evaluating at (''x''<sub>1</sub>, ''y''<sub>1</sub>) determines the value of ''c'' and the result is that the equation of the tangent is
:<math>(x_1-a)x+(y_1-b)y = (x_1-a)x_1+(y_1-b)y_1\,</math>
or
:<math>(x_1-a)(x-a)+(y_1-b)(y-b) = r^2.\!\ </math>

If {{nowrap|''y''<sub>1</sub> ≠ ''b''}} then the slope of this line is
:<math>\frac{dy}{dx} = -\frac{x_1-a}{y_1-b}.</math>

This can also be found using [[implicit differentiation]].

When the centre of the circle is at the origin then the equation of the tangent line becomes
:<math>x_1x+y_1y = r^2,\!\ </math>
and its slope is
:<math>\frac{dy}{dx} = -\frac{x_1}{y_1}.</math>

==Properties==
*The circle is the shape with the largest area for a given length of perimeter. (See [[Isoperimetric inequality]].)
*The circle is a highly symmetric shape: every line through the centre forms a line of [[reflection symmetry]] and it has [[rotational symmetry]] around the centre for every angle. Its [[symmetry group]] is the [[orthogonal group]] O(2,''R''). The group of rotations alone is the [[circle group]] '''T'''.
*All circles are [[Similarity (geometry)|similar]].
**A circle's circumference and radius are [[Proportionality (mathematics)|proportional]].
**The [[area (geometry)|area]] enclosed and the square of its radius are [[Proportionality (mathematics)|proportional]].
**The [[mathematical constant|constants]] of proportionality are 2{{pi}} and {{pi}}, respectively.
*The circle which is centred at the origin with radius 1 is called the [[unit circle]].
**Thought of as a [[great circle]] of the [[unit sphere]], it becomes the [[Riemannian circle]].
*Through any three points, not all on the same line, there lies a unique circle. In [[Cartesian coordinates]], it is possible to give explicit formulae for the coordinates of the centre of the circle and the radius in terms of the coordinates of the three given points. See [[circumcircle]].

===Chord===
*Chords are equidistant from the centre of a circle if and only if they are equal in length.
*The [[perpendicular bisector]] of a chord passes through the centre of a circle; equivalent statements stemming from the uniqueness of the perpendicular bisector are:
**A perpendicular line from the centre of a circle bisects the chord.
**The [[line segment]] through the centre bisecting a chord is [[perpendicular]] to the chord.
*If a central angle and an [[inscribed angle]] of a circle are subtended by the same chord and on the same side of the chord, then the central angle is twice the inscribed angle.
*If two angles are inscribed on the same chord and on the same side of the chord, then they are equal.
*If two angles are inscribed on the same chord and on opposite sides of the chord, then they are [[supplementary angles|supplementary]].
**For a [[cyclic quadrilateral]], the [[exterior angle]] is equal to the interior opposite angle.
*An inscribed angle subtended by a diameter is a right angle (see [[Thales' theorem]]).
*The diameter is the longest chord of the circle.
*If the intersection of any two chords divides one chord into lengths ''a'' and ''b'' and divides the other chord into lengths ''c'' and ''d'', then {{nowrap|''ab'' {{=}} ''cd''}}.
*If the intersection of any two perpendicular chords divides one chord into lengths ''a'' and ''b'' and divides the other chord into lengths ''c'' and ''d'', then {{nowrap|''a''<sup>2</sup> + ''b''<sup>2</sup> + ''c''<sup>2</sup> + ''d''<sup>2</sup>}} equals the square of the diameter.<ref>Posamentier and Salkind, ''Challenging Problems in Geometry'', Dover, 2nd edition, 1996: pp. 104–105, #4–23.</ref>
*The sum of the squared lengths of any two chords intersecting at right angles at a given point is the same as that of any other two perpendicular chords intersecting at the same point, and is given by 8''r'' <sup>2</sup> – 4''p'' <sup>2</sup> (where ''r'' is the circle's radius and ''p'' is the distance from the center point to the point of intersection).<ref>''[[College Mathematics Journal]]'' 29(4), September 1998, p. 331, problem 635.</ref>
*The distance from a point on the circle to a given chord times the diameter of the circle equals the product of the distances from the point to the ends of the chord.<ref>Johnson, Roger A., ''Advanced Euclidean Geometry'', Dover Publ., 2007.</ref>{{rp|p.71}}

===Sagitta===
[[Image:circle Sagitta.svg|thumb|277px|right|The sagitta is the vertical segment.]]
*The [[Sagitta (geometry)|sagitta]] (also known as the [[versine]]) is a line segment drawn perpendicular to a chord, between the midpoint of that chord and the arc of the circle.
*Given the length ''y'' of a chord, and the length ''x'' of the sagitta, the [[Pythagorean theorem]] can be used to calculate the radius of the unique circle which will fit around the two lines:
:: <math>r=\frac{y^2}{8x}+ \frac{x}{2}.</math>

Another proof of this result which relies only on two chord properties given above is as follows. Given a chord of length ''y'' and with sagitta of length ''x'', since the sagitta intersects the midpoint of the chord, we know it is part of a diameter of the circle. Since the diameter is twice the radius, the "missing" part of the diameter is ({{nowrap|2''r'' − ''x''}}) in length. Using the fact that one part of one chord times the other part is equal to the same product taken along a chord intersecting the first chord, we find that ({{nowrap|2''r'' − ''x'')''x'' {{=}} (''y'' / 2)<sup>2</sup>}}. Solving for ''r'', we find the required result.

===Tangent===
*A line drawn perpendicular to a radius through the end point of the radius lying on the circle is a tangent to the circle.
*A line drawn perpendicular to a tangent through the point of contact with a circle passes through the centre of the circle.
*Two tangents can always be drawn to a circle from any point outside the circle, and these tangents are equal in length.
*If a tangent at ''A'' and a tangent at ''B'' intersect at the exterior point ''P'', then denoting the centre as ''O'', the angles ∠''BOA'' and ∠''BPA'' are [[Supplementary angles|supplementary]].
*If ''AD'' is tangent to the circle at ''A'' and if ''AQ'' is a chord of the circle, then {{nowrap|∠''DAQ'' {{=}} {{frac|1|2}}arc(''AQ'')}}.

===Theorems===
[[Image:Secant-Secant Theorem.svg|thumb|right|Secant-secant theorem]]
{{See also|Power of a point}}
*The chord theorem states that if two chords, ''CD'' and ''EB'', intersect at ''A'', then {{nowrap|''CA'' × ''DA'' {{=}} ''EA'' × ''BA''}}.
*If a [[tangent]] from an external point ''D'' meets the circle at ''C'' and a [[Secant line|secant]] from the external point ''D'' meets the circle at ''G'' and ''E'' respectively, then {{nowrap|''DC''<sup>2</sup> {{=}} ''DG'' × ''DE''}}. (Tangent-secant theorem.)
*If two secants, ''DG'' and ''DE'', also cut the circle at ''H'' and ''F'' respectively, then {{nowrap|''DH'' × ''DG'' {{=}} ''DF'' × ''DE''}}. (Corollary of the tangent-secant theorem.)
*The angle between a tangent and chord is equal to one half the subtended angle on the opposite side of the chord (Tangent Chord Angle).
*If the angle subtended by the chord at the centre is 90 [[Degree (angle)|degrees]] then {{nowrap|''l'' {{=}} ''r'' √2}}, where ''l'' is the length of the chord and ''r'' is the radius of the circle.
*If two secants are inscribed in the circle as shown at right, then the measurement of angle ''A'' is equal to one half the difference of the measurements of the enclosed arcs (''DE'' and ''BC''). This is the secant-secant theorem.

===Inscribed angles===
{{See also|Inscribed angle theorem}}
[[Image:inscribed angle theorem.svg|thumb|200px|right|Inscribed angle theorem]]
An [[inscribed angle]] (examples are the blue and green angles in the figure) is exactly half the corresponding [[central angle]] (red). Hence, all inscribed angles that subtend the same arc (pink) are equal. Angles inscribed on the arc (brown) are supplementary. In particular, every inscribed angle that subtends a [[diameter]] is a [[right angle]] (since the central angle is 180 degrees).

{{clr}}

==Circle of Apollonius==
<!--This section is linked from [[Sphere]] among others-->

[[Image:Apollonius circle definition labels.svg|thumb|250px|left|Apollonius' definition of a circle: {{nowrap|''d''<sub>1</sub> / ''d''<sub>2</sub>}} constant]]
[[Apollonius of Perga]] showed that a circle may also be defined as the set of points in a plane having a constant ''ratio'' (other than 1) of distances to two fixed foci, ''A'' and ''B''.<ref>{{cite book |author=Harkness, James |title=Introduction to the theory of analytic functions |location=London, New York |publisher=Macmillan and Co. |year=1898 |page=30 |url=http://dlxs2.library.cornell.edu/cgi/t/text/text-idx?c=math;idno=01680002}}</ref><ref>[[C. Stanley Ogilvy|Ogilvy, C. Stanley]], ''Excursions in Geometry'', Dover, 1969, 14–17.</ref> (The set of points where the distances are equal is the perpendicular bisector of ''A'' and ''B'', a line.) That circle is sometimes said to be drawn ''about'' two points.

The proof is in two parts. First, one must prove that, given two foci ''A'' and ''B'' and a ratio of distances, any point ''P'' satisfying the ratio of distances must fall on a particular circle. Let ''C'' be another point, also satisfying the ratio and lying on segment ''AB''. By the [[angle bisector theorem]] the line segment ''PC'' will bisect the [[interior angle]] ''APB'', since the segments are similar:
:<math>\frac{AP}{BP} = \frac{AC}{BC}.</math>

Analogously, a line segment ''PD'' through some point ''D'' on ''AB'' extended bisects the corresponding [[exterior angle]] ''BPQ'' where ''Q'' is on ''AP'' extended. Since the interior and exterior angles sum to 180 degrees, the angle ''CPD'' is exactly 90 degrees, i.e., a [[right angle]]. The set of points ''P'' such that angle ''CPD'' is a right angle forms a circle, of which ''CD'' is a diameter.

Second, see<ref>Altshiller-Court, Nathan, ''College Geometry'', Dover, 2007 (orig. 1952).</ref>{{rp|p.15}} for a proof that every point on the indicated circle satisfies the given ratio.

===Cross-ratios===
A closely related property of circles involves the geometry of the [[cross-ratio]] of points in the [[complex plane]]. If ''A'', ''B'', and ''C'' are as above, then the circle of Apollonius for these three points is the collection of points ''P'' for which the absolute value of the cross-ratio is equal to one:
:<math>|[A,B;C,P]| = 1.\ </math>

Stated another way, ''P'' is a point on the circle of Apollonius if and only if the cross-ratio [''A'',''B'';''C'',''P''] is on the [[unit circle]] in the complex plane.

==={{anchor|Generalized circles}} Generalised circles===
{{See also|Generalised circle}}
If ''C'' is the [[midpoint]] of the segment ''AB'', then the collection of points ''P'' satisfying the Apollonius condition
:<math>\frac{|AP|}{|BP|} = \frac{|AC|}{|BC|}</math>{{pad|1em}}

is not a circle, but rather a line.

Thus, if ''A'', ''B'', and ''C'' are given distinct points in the plane, then the locus of points ''P'' satisfying the above equation is called a "generalised circle." It may either be a true circle or a line. In this sense a [[line (geometry)|line]] is a generalised circle of infinite radius.

==Circles inscribed in or circumscribed about other figures==
In every [[triangle]] a unique circle, called the [[Incircle and excircles of a triangle|incircle]], can be inscribed such that it is [[tangent]] to each of the three sides of the triangle.<ref>[http://mathworld.wolfram.com/Incircle.html Incircle – from Wolfram MathWorld]. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.</ref>

About every triangle a unique circle, called the [[circumcircle]], can be circumscribed such that it goes through each of the triangle's three [[Vertex (geometry)|vertices]].<ref>[http://mathworld.wolfram.com/Circumcircle.html Circumcircle – from Wolfram MathWorld]. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.</ref>

A tangential polygon, such as a [[tangential quadrilateral]], is any [[convex polygon]] within which a circle can be inscribed that is tangent to each side of the polygon.<ref>[http://mathworld.wolfram.com/TangentialPolygon.html Tangential Polygon – from Wolfram MathWorld]. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.</ref>

A [[cyclic polygon]] is any convex polygon about which a circle can be circumscribed, passing through each vertex. A well-studied example is the [[cyclic quadrilateral]].

A [[hypocycloid]] is a curve that is inscribed in a given circle by tracing a fixed point on a smaller circle that rolls within and tangent to the given circle.

==Circle as limiting case of other figures==
The circle can be viewed as a [[Limiting case (mathematics)|limiting case]] of each of various other figures:
*A [[Cartesian oval]] is a set of points such that a [[weighted sum]] of the distances from any of its points to two fixed points ([[Focus (geometry)|foci]]) is a constant. An [[ellipse]] is the case in which the weights are equal. A circle is an ellipse with an [[Eccentricity (mathematics)|eccentricity]] of zero, meaning that the two foci coincide with each other as the centre of the circle. A circle is also a different special case of a Cartesian oval in which one of the weights is zero.
*A [[superellipse]] has an equation of the form <math>\left|\frac{x}{a}\right|^n\! + \left|\frac{y}{b}\right|^n\! = 1</math> for positive ''a'', ''b'', and ''n''. A supercircle has {{nowrap|''b'' {{=}} ''a''}}. A circle is the special case of a supercircle in which {{nowrap|''n'' {{=}} 2}}.
*A [[Cassini oval]] is a set of points such that the product of the distances from any of its points to two fixed points is a constant. When the two fixed points coincide, a circle results.
*A [[curve of constant width]] is a figure whose width, defined as the perpendicular distance between two distinct parallel lines each intersecting its boundary in a single point, is the same regardless of the direction of those two parallel lines. The circle is the simplest example of this type of figure.

==Squaring the circle==

[[Squaring the circle]] is the problem, proposed by [[classical antiquity|ancient]] [[geometers]], of constructing a square with the same area as a given circle by using only a finite number of steps with [[compass and straightedge]].

In 1882, the task was proven to be impossible, as a consequence of the [[Lindemann–Weierstrass theorem]] which proves that [[pi]] ({{pi}}) is a [[transcendental number]], rather than an [[algebraic number|algebraic irrational number]]; that is, it is not the [[root of a function|root]] of any [[polynomial]] with [[rational number|rational]] coefficients.

==See also==
{{col-begin}}{{col-break}}

*[[Affine sphere]]
*[[Annulus (mathematics)]]
*[[Apeirogon]]
*[[Circle fitting]]
*[[Formulae of shapes]]
*[[List of circle topics]]
*[[Sphere]]
*[[Three points determine a circle]]
*[[Translation of axes]]

===Specially named circles===
*[[Unit circle]]
*[[Apollonian circles]]
*[[Chromatic circle]]
*[[Ford circle]]
*[[Circle of antisimilitude]]
*[[Carlyle circle]]
*[[Bankoff circle]]
*[[Archimedes' twin circles]]
*[[Archimedean circle]]
*[[Johnson circles]]
*[[Schoch circles]]
*[[Woo circles]]

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====Of a triangle====
*[[Mandart circle]]
*[[Spieker circle]]
*[[Nine-point circle]]
*[[Lemoine circle]]
*[[Circumcircle]]
*[[Incircle]]
*[[Excircle]]
*[[Incircle and excircles of a triangle#Other excircle properties|Apollonius circle of the excircles]]
*[[Lester circle]]
*[[Malfatti circles]]
*[[Brocard circle]]
*[[Orthocentroidal circle]]
*[[Van Lamoen circle]]
*[[Parry circle]]
*[[Polar circle (geometry)]]

{{col-break}}

====Of certain quadrilaterals====
*[[Eight-point circle]] of an orthodiagonal quadrilateral
*[[Tangential quadrilateral|Incircle of a tangential quadrilateral]]
*[[Cyclic quadrilateral|Circumcircle of a cyclic quadrilateral]]

====Of certain polygons====
*[[Cyclic polygon|Circumcircle of a cyclic polygon]]
*[[Tangential polygon|Incircle of a tangential polygon]]

====Of a conic section====
*[[Director circle]]
*[[Directrix circle]]

====Of a sphere====
*[[Great circle]]
*[[Riemannian circle]]

====Of a torus====
*[[Villarceau circles]]

{{col-end}}

==References==
{{Reflist|2}}

==Further reading==
*{{cite book |author=Pedoe, Dan |title=Geometry: a comprehensive course |publisher=Dover |year=1988}}
*[http://www-history.mcs.st-andrews.ac.uk/history/Curves/Circle.html "Circle" in The MacTutor History of Mathematics archive]

==External links==
{{Commons and category|Circles|Circles}}
{{wikiquote|Circles}}
* {{springer|title=Circle|id=p/c022260}}
*[[planetmath:4236|Circle (PlanetMath.org website)]]
*{{MathWorld |urlname=Circle |title=Circle}}
*[http://www.mathopenref.com/tocs/circlestoc.html Interactive Java applets] for the properties of and elementary constructions involving circles.
*[http://www.mathwarehouse.com/geometry/circle/interactive-circle-equation.php Interactive Standard Form Equation of Circle] Click and drag points to see standard form equation in action
*[http://www.cut-the-knot.org/pythagoras/Munching/circle.shtml Munching on Circles] at [[cut-the-knot]]
*[http://www.area-of-a-circle.com Area of a Circle] Calculate the basic properties of a circle.
*[http://www.mathace.net/wiki/index.php?title=Circle MathAce's Circle article] – has a good in-depth explanation of unit circles and transforming circular equations.
*[http://www.areacircle.com How to find the area of a circle]. There are many types of problems involving how to find the area of circle; for example, finding area of a circle from its radius, diameter or circumference.

{{Authority control}}
[[Category:Circles| ]]
[[Category:Elementary shapes]]
[[Category:Curves]]
[[Category:Conic sections]]
[[Category:Pi]]

Revision as of 15:15, 1 October 2015

This article is about the shape and mathematical concept. For other uses, see Circle (disambiguation).

A circle is circular in form.

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