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In geometry, **Descartes' theorem** states that for every four kissing, or mutually tangent, circles, the radii of the circles satisfy a certain quadratic equation. By solving this equation, one can construct a fourth circle tangent to three given, mutually tangent circles. The theorem is named after René Descartes, who stated it in 1643.

**Geometry** is a branch of mathematics concerned with questions of shape, size, relative position of figures, and the properties of space. A mathematician who works in the field of geometry is called a geometer.

In geometry, the **tangent line** to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. More precisely, a straight line is said to be a tangent of a curve *y* = *f* (*x*) at a point *x* = *c* on the curve if the line passes through the point (*c*, *f* ) on the curve and has slope *f*'(*c*) where *f*' is the derivative of *f*. A similar definition applies to space curves and curves in *n*-dimensional Euclidean space.

A **circle** is a simple closed shape. 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 radius. This article is about circles in Euclidean geometry, and, in particular, the Euclidean plane, except where otherwise noted.

Geometrical problems involving tangent circles have been pondered for millennia. In ancient Greece of the third century BC, Apollonius of Perga devoted an entire book to the topic.

**Apollonius of Perga** was a Greek geometer and astronomer known for his theories on the topic of conic sections. Beginning from the theories of Euclid and Archimedes on the topic, he brought them to the state they were in just before the invention of analytic geometry. His definitions of the terms ellipse, parabola, and hyperbola are the ones in use today.

René Descartes discussed the problem briefly in 1643, in a letter to Princess Elisabeth of the Palatinate. He came up with essentially the same solution as given in ** equation (1) ** below, and thus attached his name to the theorem.

**René Descartes** was a French philosopher, mathematician, and scientist. A native of the Kingdom of France, he spent about 20 years (1629–1649) of his life in the Dutch Republic after serving for a while in the Dutch States Army of Maurice of Nassau, Prince of Orange and the Stadtholder of the United Provinces. He is generally considered one of the most notable intellectual figures of the Dutch Golden Age.

**Elisabeth of the Palatinate**, also known as **Elisabeth of Bohemia**, Princess Elisabeth of the Palatinate, or Princess-Abbess of Herford Abbey, was the eldest daughter of Frederick V, Elector Palatine, and Elizabeth Stuart. Elisabeth of the Palatinate is a philosopher best known for her correspondence with René Descartes. She was critical of Descartes' dualistic metaphysics and her work anticipated the metaphysical concerns of later philosophers.

Frederick Soddy rediscovered the equation in 1936. The kissing circles in this problem are sometimes known as **Soddy circles**, perhaps because Soddy chose to publish his version of the theorem in the form of a poem titled *The Kiss Precise*, which was printed in *Nature* (June 20, 1936). Soddy also extended the theorem to spheres; Thorold Gosset extended the theorem to arbitrary dimensions.

**Frederick Soddy** FRS was an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also proved the existence of isotopes of certain radioactive elements.

* Nature* is a British multidisciplinary scientific journal, first published on 4 November 1869. It is one of the most recognizable scientific journals in the world, and was ranked the world's most cited scientific journal by the Science Edition of the 2010

**John Herbert de Paz Thorold Gosset** was an English lawyer and an amateur mathematician. In mathematics, he is noted for discovering and classifying the semiregular polytopes in dimensions four and higher.

Descartes' theorem is most easily stated in terms of the circles' curvatures. The **curvature** (or **bend**) of a circle is defined as *k* = ±1/*r*, where *r* is its radius. The larger a circle, the smaller is the magnitude of its curvature, and vice versa.

In mathematics, **curvature** is any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object such as a surface deviates from being a *flat* plane, or a curve from being *straight* as in the case of a line, but this is defined in different ways depending on the context. There is a key distinction between *extrinsic curvature*, which is defined for objects embedded in another space – in a way that relates to the radius of curvature of circles that touch the object – and *intrinsic curvature*, which is defined in terms of the lengths of curves within a Riemannian manifold.

In mathematics, **magnitude** is the size of a mathematical object, a property which determines whether the object is larger or smaller than other objects of the same kind. More formally, an object's magnitude is the displayed result of an ordering of the class of objects to which it belongs.

The plus sign in *k* = ±1/*r* applies to a circle that is *externally* tangent to the other circles, like the three black circles in the image. For an *internally* tangent circle like the big red circle, that *circumscribes* the other circles, the minus sign applies.

If a straight line is considered a degenerate circle with zero curvature (and thus infinite radius), Descartes' theorem also applies to a line and two circles that are all three mutually tangent, giving the radius of a third circle tangent to the other two circles and the line.

In mathematics, a **degenerate case** is a limiting case in which an element of a class of objects is qualitatively different from the rest of the class and hence belongs to another, usually simpler, class. **Degeneracy** is the condition of being a degenerate case.

If four circles are tangent to each other at six distinct points, and the circles have curvatures *k*_{i} (for *i* = 1, ..., 4), Descartes' theorem says:

**(1)**

When trying to find the radius of a fourth circle tangent to three given kissing circles, the equation is best rewritten as:

**(2)**

The ± sign reflects the fact that there are in general *two* solutions. Ignoring the degenerate case of a straight line, one solution is positive and the other is either positive or negative; if negative, it represents a circle that circumscribes the first three (as shown in the diagram above).

Problem-specific criteria may favor one solution over the other in any given problem.

If one of the three circles is replaced by a straight line, then one *k*_{i}, say *k*_{3}, is zero and drops out of ** equation (1) **. ** Equation (2) ** then becomes much simpler:

**(3)**

If two circles are replaced by lines, the tangency between the two replaced circles becomes a parallelism between their two replacement lines. For all four curves to remain mutually tangent, the other two circles must be congruent. In this case, with *k*_{2} = *k*_{3} = 0, ** equation (2) ** is reduced to the trivial

It is not possible to replace three circles by lines, as it is not possible for three lines and one circle to be mutually tangent. Descartes' theorem does not apply when all four circles are tangent to each other at the same point.

Another special case is when the *k _{i}* are squares,

Euler showed that this is equivalent to the simultaneous triplet of Pythagorean triples,

and can be given a parametric solution. When the minus sign of a curvature is chosen,

this can be solved^{ [1] } as,

where

parametric solutions of which are well-known.

To determine a circle completely, not only its radius (or curvature), but also its center must be known. The relevant equation is expressed most clearly if the coordinates (*x*, *y*) are interpreted as a complex number *z* = *x* + i*y*. The equation then looks similar to Descartes' theorem and is therefore called the **complex Descartes theorem**.

Given four circles with curvatures *k*_{i} and centers *z*_{i} (for *i* = 1...4), the following equality holds in addition to ** equation (1) **:

**(4)**

Once *k*_{4} has been found using ** equation (2) **, one may proceed to calculate *z*_{4} by rewriting ** equation (4) ** to a form similar to ** equation (2) **:

Again, in general, there are two solutions for *z*_{4}, corresponding to the two solutions for *k*_{4}. Note that the plus/minus sign in the above formula for z does not necessarily correspond to the plus/minus sign in the formula for k.

The generalization to n dimensions is sometimes referred to as the **Soddy–Gosset theorem**, even though it was shown by R. Lachlan in 1886. In *n*-dimensional Euclidean space, the maximum number of mutually tangent (*n*− 1)-spheres is *n* + 2. For example, in 3-dimensional space, five spheres can be mutually tangent.The curvatures of the hyperspheres satisfy

with the case *k _{i}* = 0 corresponding to a flat hyperplane, in exact analogy to the 2-dimensional version of the theorem.

Although there is no 3-dimensional analogue of the complex numbers, the relationship between the positions of the centers can be re-expressed as a matrix equation, which also generalizes to *n* dimensions.^{ [2] }

- ↑ A Collection of Algebraic Identities: Sums of Three or More 4th Powers
- ↑ Jeffrey C. Lagarias; Colin L. Mallows; Allan R. Wilks (April 2002). "Beyond the Descartes Circle Theorem".
*The American Mathematical Monthly*.**109**(4): 338–361. arXiv: math/0101066 . doi:10.2307/2695498. JSTOR 2695498.

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In classical mathematics, **analytic geometry**, also known as **coordinate geometry** or **Cartesian geometry**, is the study of geometry using a coordinate system. This contrasts with synthetic geometry.

In mathematics, a **parabola** is a plane curve that is mirror-symmetrical and is approximately U-shaped. It fits several superficially different other mathematical descriptions, which can all be proved to define exactly the same curves.

A **sphere** is a perfectly round geometrical object in three-dimensional space that is the surface of a completely round ball.

**Bézout's theorem** is a statement in algebraic geometry concerning the number of common points, or intersection points, of two plane algebraic curves which do not share a common component. The theorem states that the number of common points of two such curves is at most equal to the product of their degrees, and equality holds if one counts points at infinity and points with complex coordinates, and if each point is counted with its intersection multiplicity. It is named after Étienne Bézout.

In geometry, the **incircle** or **inscribed circle** of a triangle is the largest circle contained in the triangle; it touches the three sides. The center of the incircle is a triangle center called the triangle's incenter.

In differential geometry, the **Ricci curvature tensor**, named after Gregorio Ricci-Curbastro, represents the amount by which the volume of a narrow conical piece of a small geodesic ball in a curved Riemannian manifold deviates from that of the standard ball in Euclidean space. As such, it provides one way of measuring the degree to which the geometry determined by a given Riemannian metric might differ from that of ordinary Euclidean n-space. The Ricci tensor is defined on any pseudo-Riemannian manifold, as a trace of the Riemann curvature tensor. Like the metric itself, the Ricci tensor is a symmetric bilinear form on the tangent space of the manifold.

In differential geometry, the **Gaussian curvature** or **Gauss curvature** *Κ* of a surface at a point is the product of the principal curvatures, *κ*_{1} and *κ*_{2}, at the given point:

A **cardioid** is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spiral, and an inverse curve of the parabola with the focus as the center of inversion.

In mathematics, a **Dupin cyclide** or **cyclide of Dupin** is any geometric inversion of a standard torus, cylinder or double cone. In particular, these latter are themselves examples of Dupin cyclides. They were discovered by Charles Dupin in his 1803 dissertation under Gaspard Monge. The key property of a Dupin cyclide is that it is a channel surface in two different ways. This property means that Dupin cyclides are natural objects in Lie sphere geometry.

The **nephroid** is a plane curve whose name means 'kidney-shaped'. Although the term *nephroid* was used to describe other curves, it was applied to the curve in this article by Proctor in 1878.

In mathematics, an **Apollonian gasket** or **Apollonian net** is a fractal generated starting from a triple of circles, each tangent to the other two, and successively filling in more circles, each tangent to another three. It is named after Greek mathematician Apollonius of Perga.

In Euclidean plane geometry, **Apollonius's problem** is to construct circles that are tangent to three given circles in a plane (Figure 1). Apollonius of Perga posed and solved this famous problem in his work Ἐπαφαί ; this work has been lost, but a 4th-century AD report of his results by Pappus of Alexandria has survived. Three given circles generically have eight different circles that are tangent to them (Figure 2), a pair of solutions for each way to divide the three given circles in two subsets.

In geometry, **collinearity** of a set of points is the property of their lying on a single line. A set of points with this property is said to be **collinear**. In greater generality, the term has been used for aligned objects, that is, things being "in a line" or "in a row".

A **parametric surface** is a surface in the Euclidean space
which is defined by a parametric equation with two parameters
Parametric representation is a very general way to specify a surface, as well as implicit representation. Surfaces that occur in two of the main theorems of vector calculus, Stokes' theorem and the divergence theorem, are frequently given in a parametric form. The curvature and arc length of curves on the surface, surface area, differential geometric invariants such as the first and second fundamental forms, Gaussian, mean, and principal curvatures can all be computed from a given parametrization.

In mathematics, the **differential geometry of surfaces** deals with the differential geometry of smooth surfaces with various additional structures, most often, a Riemannian metric.
Surfaces have been extensively studied from various perspectives: *extrinsically*, relating to their embedding in Euclidean space and *intrinsically*, reflecting their properties determined solely by the distance within the surface as measured along curves on the surface. One of the fundamental concepts investigated is the Gaussian curvature, first studied in depth by Carl Friedrich Gauss, who showed that curvature was an intrinsic property of a surface, independent of its isometric embedding in Euclidean space.

In mathematics, a **conic section** is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse. The circle is a special case of the ellipse, and is of sufficient interest in its own right that it was sometimes called a fourth type of conic section. The conic sections have been studied by the ancient Greek mathematicians with this work culminating around 200 BC, when Apollonius of Perga undertook a systematic study of their properties.

In Euclidean plane geometry, a **tangent line to a circle** is a line that touches the circle at exactly one point, never entering the circle's interior. Tangent lines to circles form the subject of several theorems, and play an important role in many geometrical constructions and proofs. Since the tangent line to a circle at a point **P** is perpendicular to the radius to that point, theorems involving tangent lines often involve radial lines and orthogonal circles.