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A circle is a simple closed in Euclidean geometry. It is the set of all points in a plane that are at a given distance from a given point, the centre; 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 .

A circle is a simple closed which divides the plane into two regions: an 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 only the boundary and the whole figure is called a disk.

A circle may also be defined as a special in which the two foci are coincident and the eccentricity is 0, or the two-dimensional shape enclosing the most area per unit perimeter squared, using calculus of variations.

Terminology
• Annulus: the ring-shaped object, the region bounded by two circles.
• Arc: any part of the circle.
• Centre: the point equidistant from the points on the circle.
• Chord: a line segment whose endpoints lie on the circle.
• : the length of one circuit along the circle, or the distance around the circle.
• : 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.
• Disk: the region of the plane bounded by a circle
• Lens: the intersection of two disks
• Passant: a straight line that does not touch the circle.
• : 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.
• : a region bounded by two radii and an arc lying between the radii.
• : a region, not containing the centre, bounded by a chord and an arc lying between the chord's endpoints.
• : an extended chord, a coplanar straight line cutting the circle at two points.
• : 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. A half-disk is a special case of a , namely the largest one.
• : a coplanar straight line that touches the circle at a single point.

History
[[Image:God the Geometer.jpg|thumb|right|200px| The in this 13th-century manuscript is a symbol of God's act of . Notice also the circular shape of the halo.]] The word circle derives from the κίρκος/κύκλος ( kirkos/kuklos), itself a metathesis of the κρίκος ( krikos), meaning "hoop" or "ring". krikos, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus The origins of the words and are closely related. 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 , which, with related inventions such as , makes much of modern machinery possible. In mathematics, the study of the circle has helped inspire the development of geometry, , and calculus.

Early , particularly and astrology and astronomy, was connected to the divine for most medieval scholars, and many believed that there was something intrinsically "divine" or "perfect" that could be found in circles., : A History of Man's Changing Vision of the Universe (1959), The Six Books of Proclus, the Platonic Successor, on the Theology of Plato Tr. Thomas Taylor (1816) Vol. 2, Ch. 2, "Of Plato"

Some highlights in the history of the circle are:

• 1700 BCE – The gives a method to find the area of a circular field. The result corresponds to (3.16049...) as an approximate value of . Chronology for 30000 BC to 500 BC. History.mcs.st-andrews.ac.uk. Retrieved on 2012-05-03.
• 300 BCE – Book 3 of Euclid's Elements deals with the properties of circles.
• In 's 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 – Lindemann proves that is transcendental, effectively settling the millennia-old problem of squaring the circle. Squaring the circle. History.mcs.st-andrews.ac.uk. Retrieved on 2012-05-03.

Analytic results

Length of circumference
The ratio of a circle's to its is (pi), an irrational constant approximately equal to 3.141592654. Thus the length of the circumference C is related to the radius r and diameter d by:
$C = 2\pi r = \pi d.\,$

Area enclosed
As proved by , in his Measurement of a Circle, the 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, which comes to multiplied by the radius squared:
$\mathrm\left\{Area\right\} = \pi r^2.\,$

Equivalently, denoting diameter by d,

$\mathrm\left\{Area\right\} = \frac\left\{\pi d^2\right\}\left\{4\right\} \approx 0\left\{.\right\}7854d^2,$

that is, approximately 79 percent of the 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
In an xy Cartesian coordinate system, the circle with centre coordinates ( a, b) and radius r is the set of all points ( x, y) such that
$\left\left(x - a \right\right)^2 \left\left( y - b \right\right)^2=r^2.$

This , 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 | xa| and | yb|. If the circle is centred at the origin (0, 0), then the equation simplifies to

$x^2 y^2 = r^2.\!\$

The equation can be written in parametric form using the trigonometric functions sine and cosine as

$x = a r\,\cos t,\,$
$y = b r\,\sin t\,$
where t is a parametric variable in the range 0 to 2, interpreted geometrically as the that the ray from ( a, b) to ( x, y) makes with the positive x-axis.

An alternative parametrisation of the circle is:

$x = a r \frac\left\{2t\right\}\left\{1 t^2\right\}.\,$
$y = b r \frac\left\{1-t^2\right\}\left\{1 t^2\right\}\,$

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 with the equation of a circle has the form

$x^2 y^2-2axz-2byz cz^2 = 0.\,$

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: − i: 0). These points are called the circular points at infinity.

Polar coordinates
In polar coordinates the equation of a circle is:
$r^2 - 2 r r_0 \cos\left(\theta - \phi\right) r_0^2 = a^2\,$

where a is the radius of the circle, $\left(r, \theta\right)$ is the polar coordinate of a generic point on the circle, and $\left(r_0, \phi\right)$ is the polar coordinate of the centre of the circle (i.e., r0 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. r0 = 0, this reduces to simply . When , or when the origin lies on the circle, the equation becomes

$r = 2 a\cos\left(\theta - \phi\right).\,$

In the general case, the equation can be solved for r, giving

$r = r_0 \cos\left(\theta - \phi\right) \pm \sqrt\left\{a^2 - r_0^2 \sin^2\left(\theta - \phi\right)\right\},$
Note that without the ± sign, the equation would in some cases describe only half a circle.

Complex plane
In the , a circle with a centre at c and radius ( r) has the equation $|z-c| = r\,$. In parametric form this can be written $z = re^\left\{it\right\} c$.

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

Tangent lines
The through a point P on the circle is perpendicular to the diameter passing through P. If and the circle has centre ( a, b) and radius r, then the tangent line is perpendicular to the line from ( a, b) to ( x1, y1), so it has the form . Evaluating at ( x1, y1) determines the value of c and the result is that the equation of the tangent is
$\left(x_1-a\right)x \left(y_1-b\right)y = \left(x_1-a\right)x_1 \left(y_1-b\right)y_1\,$
or
$\left(x_1-a\right)\left(x-a\right) \left(y_1-b\right)\left(y-b\right) = r^2.\!\$

If then the slope of this line is

$\frac\left\{dy\right\}\left\{dx\right\} = -\frac\left\{x_1-a\right\}\left\{y_1-b\right\}.$

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

$x_1x y_1y = r^2,\!\$
and its slope is
$\frac\left\{dy\right\}\left\{dx\right\} = -\frac\left\{x_1\right\}\left\{y_1\right\}.$

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 is the O(2, R). The group of rotations alone is the T.
• All circles are similar.
• A circle's circumference and radius are proportional.
• The area enclosed and the square of its radius are proportional.
• The constants of proportionality are 2 and , respectively.
• The circle which is centred at the origin with radius 1 is called the .
• 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 .

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 through the centre bisecting a chord is to the chord.
• If a central angle and an 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.
• For a cyclic quadrilateral, the 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 .
• 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 equals the square of the diameter.Posamentier and Salkind, Challenging Problems in Geometry, Dover, 2nd edition, 1996: pp. 104–105, #4–23.
• 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 2 – 4 p 2 (where r is the circle's radius and p is the distance from the centre point to the point of intersection). College Mathematics Journal 29(4), September 1998, p. 331, problem 635.
• 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.Johnson, Roger A., Advanced Euclidean Geometry, Dover Publ., 2007.

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.
• If AD is tangent to the circle at A and if AQ is a chord of the circle, then .

Theorems
• The chord theorem states that if two chords, CD and EB, intersect at A, then .
• If a from an external point D meets the circle at C and a from the external point D meets the circle at G and E respectively, then . (Tangent-secant theorem.)
• If two secants, DG and DE, also cut the circle at H and F respectively, then . (Corollary of the tangent-secant theorem.)
• The angle between a tangent and chord is equal to one half the angle subtended at the center of the circle, on the opposite side of the chord (Tangent Chord Angle).
• If the angle subtended by the chord at the centre is 90 degrees then , 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
An (examples are the blue and green angles in the figure) is exactly half the corresponding (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 is a (since the central angle is 180 degrees).

Sagitta
• The sagitta (also known as the ) 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:
: $r=\frac\left\{y^2\right\}\left\{8x\right\} \frac\left\{x\right\}\left\{2\right\}.$

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 () 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 (. Solving for r, we find the required result.

Compass and straightedge constructions
There are many compass-and-straightedge constructions resulting in circles.

The simplest and most basic is the construction given the centre of the circle and a point on the circle. Place the fixed leg of the compass on the centre point, the movable leg on the point on the circle and rotate the compass.

Construct a circle with a given diameter
• Construct the midpoint of the diameter.
• Construct the circle with centre passing through one of the endpoints of the diameter (it will also pass through the other endpoint).

Construct a circle through 3 noncollinear points
• Name the points , and ,
• Construct the perpendicular bisector of the segment .
• Construct the perpendicular bisector of the segment .
• Label the point of intersection of these two perpendicular bisectors . (They meet because the points are not ).
• Construct the circle with centre passing through one of the points , or (it will also pass through the other two points).

Circle of Apollonius
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.Ogilvy, C. Stanley, Excursions in Geometry, Dover, 1969, 14–17. (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 APB, since the segments are similar:

$\frac\left\{AP\right\}\left\{BP\right\} = \frac\left\{AC\right\}\left\{BC\right\}.$

Analogously, a line segment PD through some point D on AB extended bisects the corresponding 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 . The set of points P such that angle CPD is a right angle forms a circle, of which CD is a diameter.

Second, seeAltshiller-Court, Nathan, College Geometry, Dover, 2007 (orig. 1952). 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 of points in the . 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:
$|A,B;C,P| = 1.\$

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 in the complex plane.

Generalised circles
If C is the of the segment AB, then the collection of points P satisfying the Apollonius condition
$\frac$
= \frac

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 is a generalised circle of infinite radius.

Circles inscribed in or circumscribed about other figures
In every a unique circle, called the incircle, can be inscribed such that it is to each of the three sides of the triangle. Incircle – from Wolfram MathWorld. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.

About every triangle a unique circle, called the , can be circumscribed such that it goes through each of the triangle's three vertices. Circumcircle – from Wolfram MathWorld. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.

A tangential polygon, such as a tangential quadrilateral, is any within which a circle can be inscribed that is tangent to each side of the polygon. Tangential Polygon – from Wolfram MathWorld. Mathworld.wolfram.com (2012-04-26). Retrieved on 2012-05-03.

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

A 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 of each of various other figures:
• A is a set of points such that a of the distances from any of its points to two fixed points (foci) is a constant. An is the case in which the weights are equal. A circle is an ellipse with an 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 has an equation of the form $\left|\frac\left\{x\right\}\left\{a\right\}\right|^n\! \left|\frac\left\{y\right\}\left\{b\right\}\right|^n\! = 1$ for positive a, b, and n. A supercircle has . A circle is the special case of a supercircle in which .
• A 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 ancient , 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 () is a transcendental number, rather than an ; that is, it is not the root of any with coefficients.

Specially named circles

Of a triangle

• Eight-point circle of an orthodiagonal quadrilateral
• Incircle of a tangential quadrilateral
• Circumcircle of a cyclic quadrilateral

Of certain polygons

Of a conic section

Of a sphere

Of a torus
• Villarceau circles

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