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In mathematics, a **conic section** (or simply **conic**) 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, though historically it was sometimes called a fourth type. The ancient Greek mathematicians studied conic sections, culminating around 200 BC with Apollonius of Perga's systematic work on their properties.

**Mathematics** includes the study of such topics as quantity, structure (algebra), space (geometry), and change. It has no generally accepted definition.

In mathematics, a **curve** is, generally speaking, an object similar to a line but that need not be straight. Thus, a curve is a generalization of a line, in that it may be curved.

In geometry, a (**general**) **conical surface** is the unbounded surface formed by the union of all the straight lines that pass through a fixed point — the *apex* or *vertex* — and any point of some fixed space curve — the *directrix* — that does not contain the apex. Each of those lines is called a *generatrix* of the surface.

- Euclidean geometry
- Definition
- Eccentricity, focus and directrix
- Conic parameters
- Standard forms in Cartesian coordinates
- General Cartesian form
- Polar coordinates
- Properties
- History
- Menaechmus and early works
- Apollonius of Perga
- Al-Kuhi
- Omar Khayyám
- Europe
- Applications
- In the real projective plane
- Intersection at infinity
- Homogeneous coordinates
- Projective definition of a circle
- Steiner's projective conic definition
- Line conics
- Von Staudt's definition
- Constructions
- In the complex projective plane
- Degenerate cases
- Pencil of conics
- Intersecting two conics
- Generalizations
- In other areas of mathematics
- See also
- Notes
- References
- External links

The conic sections in the Euclidean plane have various distinguishing properties, many of which can be used as alternative definitions. One such property defines a non-circular conic^{ [1] } to be the set of those points whose distances to some particular point, called a * focus *, and some particular line, called a *directrix*, are in a fixed ratio, called the * eccentricity *. The type of conic is determined by the value of the eccentricity. In analytic geometry, a conic may be defined as a plane algebraic curve of degree 2; that is, as the set of points whose coordinates satisfy a quadratic equation in two variables. This equation may be written in matrix form, and some geometric properties can be studied as algebraic conditions.

In geometry, **focuses** or **foci**, singular **focus**, are special points with reference to which any of a variety of curves is constructed. For example, one or two foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola. In addition, two foci are used to define the Cassini oval and the Cartesian oval, and more than two foci are used in defining an n-ellipse.

In mathematics, the **eccentricity** of a conic section is a non-negative real number that uniquely characterizes its shape.

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 the Euclidean plane, the three types of conic sections appear quite different, but share many properties. By extending the Euclidean plane to include a line at infinity, obtaining a projective plane, the apparent difference vanishes: the branches of a hyperbola meet in two points at infinity, making it a single closed curve; and the two ends of a parabola meet to make it a closed curve tangent to the line at infinity. Further extension, by expanding the real coordinates to admit complex coordinates, provides the means to see this unification algebraically.

In mathematics, a **projective plane** is a geometric structure that extends the concept of a plane. In the ordinary Euclidean plane, two lines typically intersect in a single point, but there are some pairs of lines that do not intersect. A projective plane can be thought of as an ordinary plane equipped with additional "points at infinity" where parallel lines intersect. Thus *any* two distinct lines in a projective plane intersect in one and only one point.

In mathematics, a **real number** is a value of a continuous quantity that can represent a distance along a line. The adjective *real* in this context was introduced in the 17th century by René Descartes, who distinguished between real and imaginary roots of polynomials. The real numbers include all the rational numbers, such as the integer −5 and the fraction 4/3, and all the irrational numbers, such as √2. Included within the irrationals are the transcendental numbers, such as π (3.14159265...). In addition to measuring distance, real numbers can be used to measure quantities such as time, mass, energy, velocity, and many more.

A **complex number** is a number that can be expressed in the form *a* + *bi*, where *a* and *b* are real numbers, and *i* is a solution of the equation *x*^{2} = −1. Because no real number satisfies this equation, *i* is called an imaginary number. For the complex number *a* + *bi*, *a* is called the **real part**, and *b* is called the **imaginary part**. Despite the historical nomenclature "imaginary", complex numbers are regarded in the mathematical sciences as just as "real" as the real numbers, and are fundamental in many aspects of the scientific description of the natural world.

The conic sections have been studied for thousands of years and have provided a rich source of interesting and beautiful results in Euclidean geometry.

**Euclidean geometry** is a mathematical system attributed to Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the *Elements*. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions (theorems) from these. Although many of Euclid's results had been stated by earlier mathematicians, Euclid was the first to show how these propositions could fit into a comprehensive deductive and logical system. The *Elements* begins with plane geometry, still taught in secondary school as the first axiomatic system and the first examples of formal proof. It goes on to the solid geometry of three dimensions. Much of the *Elements* states results of what are now called algebra and number theory, explained in geometrical language.

A **conic** is the curve obtained as the intersection of a plane, called the *cutting plane*, with the surface of a double cone (a cone with two *nappes*). It shall be assumed that the cone is a right circular cone for the purpose of easy description, but this is not required; any double cone with some circular cross-section will suffice. Planes that pass through the vertex of the cone will intersect the cone in a point, a line or a pair of intersecting lines. These are called ** degenerate conics ** and some authors do not consider them to be conics at all. Unless otherwise stated, "conic" in this article will refer to a non-degenerate conic.

In mathematics, a *plane* is a flat, two-dimensional surface that extends infinitely far. A plane is the two-dimensional analogue of a point, a line and three-dimensional space. Planes can arise as subspaces of some higher-dimensional space, as with a room's walls extended infinitely far, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry.

There are three types of conics: the ellipse, parabola, and hyperbola. The circle is a special kind of ellipse, although historically Apollonius considered as a fourth type. Ellipses arise when the intersection of the cone and plane is a closed curve. The circle is obtained when the cutting plane is parallel to the plane of the generating circle of the cone; for a right cone, this means the cutting plane is perpendicular to the axis. If the cutting plane is parallel to exactly one generating line of the cone, then the conic is unbounded and is called a *parabola*. In the remaining case, the figure is a *hyperbola*: the plane intersects *both* halves of the cone, producing two separate unbounded curves.

In mathematics, an **ellipse** is a plane curve surrounding two focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. As such, it generalizes a circle, which is the special type of ellipse in which the two focal points are the same. The elongation of an ellipse is measured by its eccentricity *e*, a number ranging from *e =* 0 to *e* = 1.

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

In mathematics, a **hyperbola** is a type of smooth curve lying in a plane, defined by its geometric properties or by equations for which it is the solution set. A hyperbola has two pieces, called connected components or branches, that are mirror images of each other and resemble two infinite bows. The hyperbola is one of the three kinds of conic section, formed by the intersection of a plane and a double cone. If the plane intersects both halves of the double cone but does not pass through the apex of the cones, then the conic is a hyperbola.

Alternatively, one can define a conic section purely in terms of plane geometry: it is the locus of all points P whose distance to a fixed point F (called the **focus**) is a constant multiple (called the **eccentricity** e) of the distance from P to a fixed line L (called the **directrix**). For 0 < *e* < 1 we obtain an ellipse, for *e* = 1 a parabola, and for *e* > 1 a hyperbola.

A circle is a limiting case and is not defined by a focus and directrix in the Euclidean plane. The eccentricity of a circle is defined to be zero and its focus is the center of the circle, but its directrix can only be taken as the line at infinity in the projective plane.^{ [2] }

The eccentricity of an ellipse can be seen as a measure of how far the ellipse deviates from being circular.

If the angle between the surface of the cone and its axis is and the angle between the cutting plane and the axis is the eccentricity is^{ [3] }

A proof that the above curves defined by the focus-directrix property are the same as those obtained by planes intersecting a cone is facilitated by the use of Dandelin spheres.^{ [4] }

In addition to the eccentricity (*e*), foci, and directrix, various geometric features and lengths are associated with a conic section.

The **principal axis** is the line joining the foci of an ellipse or hyperbola, and its midpoint is the curve's **center**. A parabola has no center.

The **linear eccentricity** (*c*) is the distance between the center and a focus.

The **latus rectum** is the chord parallel to the directrix and passing through a focus; its half-length is the semi-latus rectum (*ℓ*).

The **focal parameter** (*p*) is the distance from a focus to the corresponding directrix.

The **major axis** is the chord between the two vertices: the longest chord of an ellipse, the shortest chord between the branches of a hyperbola. Its half-length is the semi-major axis (a). When an ellipse or hyperbola are in standard position as in the equations below, with foci on the x-axis and center at the origin, the vertices of the conic have coordinates (−*a*, 0) and (*a*, 0), with a non-negative.

The **minor axis** is the shortest chord of an ellipse, and its half-length is the semi-minor axis (*b*), the same value *b* as in the standard equation below. By analogy, for a hyperbola we also call the parameter *b* in the standard equation the semi-minor axis.

The following relations hold:^{ [5] }

For conics in standard position, these parameters have the following values, taking .

conic section | equation | eccentricity (e) | linear eccentricity (c) | semi-latus rectum (ℓ) | focal parameter (p) |
---|---|---|---|---|---|

circle | |||||

ellipse | |||||

parabola | N/A | ||||

hyperbola |

After introducing Cartesian coordinates, the focus-directrix property can be used to produce the equations satisfied by the points of the conic section.^{ [6] } By means of a change of coordinates (rotation and translation of axes) these equations can be put into *standard forms*.^{ [7] } For ellipses and hyperbolas a standard form has the x-axis as principal axis and the origin (0,0) as center. The vertices are (±*a*, 0) and the foci (±*c*, 0). Define b by the equations *c*^{2} = *a*^{2} − *b*^{2} for an ellipse and *c*^{2} = *a*^{2} + *b*^{2} for a hyperbola. For a circle, *c* = 0 so *a*^{2} = *b*^{2}. For the parabola, the standard form has the focus on the x-axis at the point (*a*, 0) and the directrix the line with equation *x* = −*a*. In standard form the parabola will always pass through the origin.

For a **rectangular** or **equilateral****hyperbola**, one whose asymptotes are perpendicular, there is an alternative standard form in which the asymptotes are the coordinate axes and the line *x* = *y* is the principal axis. The foci then have coordinates (*c*, *c*) and (−*c*, −*c*).^{ [8] }

- Circle:
*x*^{2}+*y*^{2}=*a*^{2} - Ellipse:
*x*^{2}/*a*^{2}+*y*^{2}/*b*^{2}= 1 - Parabola:
*y*^{2}= 4*ax*with*a*> 0 - Hyperbola:
*x*^{2}/*a*^{2}−*y*^{2}/*b*^{2}= 1 - Rectangular hyperbola:
^{ [9] }*xy*=*c*^{2}/2

The first four of these forms are symmetric about both the *x*-axis and *y*-axis (for the circle, ellipse and hyperbola), or about the *x*-axis only (for the parabola). The rectangular hyperbola, however, is instead symmetric about the lines *y* = *x* and *y* = −*x*.

These standard forms can be written parametrically as,

- Circle: (
*a*cos*θ*,*a*sin*θ*), - Ellipse: (
*a*cos*θ*,*b*sin*θ*), - Parabola: (
*at*^{2}, 2*at*), - Hyperbola: (
*a*sec*θ*,*b*tan*θ*) or (±*a*cosh*u*,*b*sinh*u*), - Rectangular hyperbola: where

In the Cartesian coordinate system, the graph of a quadratic equation in two variables is always a conic section (though it may be degenerate ^{ [10] }), and all conic sections arise in this way. The most general equation is of the form^{ [11] }

with all coefficients real numbers and A, B, C not all zero.

The above equation can be written in matrix notation as^{ [12] }

The general equation can also be written as

This form is a specialization of the homogeneous form used in the more general setting of projective geometry (see below).

The conic sections described by this equation can be classified in terms of the value , called the discriminant of the equation.^{ [13] } Thus, the discriminant is − 4Δ where Δ is the matrix determinant

If the conic is non-degenerate, then:^{ [14] }

- if
*B*^{2}− 4*AC*< 0, the equation represents an ellipse;- if
*A*=*C*and*B*= 0, the equation represents a circle, which is a special case of an ellipse;

- if
- if
*B*^{2}− 4*AC*= 0, the equation represents a parabola; - if
*B*^{2}− 4*AC*> 0, the equation represents a hyperbola;- if
*A*+*C*= 0, the equation represents a rectangular hyperbola.

- if

In the notation used here, A and B are polynomial coefficients, in contrast to some sources that denote the semimajor and semiminor axes as A and B.

The discriminant *B*^{2} – 4*AC* of the conic section's quadratic equation (or equivalently the determinant *AC* – *B*^{2}/4 of the 2×2 matrix) and the quantity *A* + *C* (the trace of the 2×2 matrix) are invariant under arbitrary rotations and translations of the coordinate axes,^{ [14] }^{ [15] }^{ [16] } as is the determinant of the 3×3 matrix above.^{ [17] }^{:pp. 60–62} The constant term *F* and the sum *D*^{2}+*E*^{2} are invariant under rotation only.^{ [17] }^{:pp. 60–62}

When the conic section is written algebraically as

the eccentricity can be written as a function of the coefficients of the quadratic equation.^{ [18] } If 4*AC* = *B*^{2} the conic is a parabola and its eccentricity equals 1 (provided it is non-degenerate). Otherwise, assuming the equation represents either a non-degenerate hyperbola or ellipse, the eccentricity is given by

where η = 1 if the determinant of the 3×3 matrix above is negative and η = −1 if that determinant is positive.

It can also be shown^{ [17] }^{:p. 89} that the eccentricity is a positive solution of the equation

where again This has precisely one positive solution—the eccentricity— in the case of a parabola or ellipse, while in the case of a hyperbola it has two positive solutions, one of which is the eccentricity.

In the case of an ellipse or hyperbola, the equation

can be converted to canonical form in transformed variables as^{ [19] }

or equivalently

where and are the eigenvalues of the matrix — that is, the solutions of the equation

— and is the determinant of the 3×3 matrix above, and is again the determinant of the 2×2 matrix. In the case of an ellipse the squares of the two semi-axes are given by the denominators in the canonical form.

In polar coordinates, a conic section with one focus at the origin and, if any, the other at a negative value (for an ellipse) or a positive value (for a hyperbola) on the *x*-axis, is given by the equation

where *e* is the eccentricity and *l* is the semi-latus rectum.

As above, for *e* = 0, the graph is a circle, for 0 < *e* < 1the graph is an ellipse, for *e* = 1 a parabola, and for *e* > 1 a hyperbola.

The polar form of the equation of a conic is often used in dynamics; for instance, determining the orbits of objects revolving about the Sun. ^{ [20] }

Just as two (distinct) points determine a line, five points determine a conic. Formally, given any five points in the plane in general linear position, meaning no three collinear, there is a unique conic passing through them, which will be non-degenerate; this is true in both the Euclidean plane and its extension, the real projective plane. Indeed, given any five points there is a conic passing through them, but if three of the points are collinear the conic will be degenerate (reducible, because it contains a line), and may not be unique; see further discussion.

Four points in the plane in general linear position determine a unique conic passing through the first three points and having the fourth point as its center. Thus knowing the center is equivalent to knowing two points on the conic for the purpose of determining the curve.^{ [21] }

Furthermore, a conic is determined by any combination of *k* points in general position that it passes through and 5 – *k* lines that are tangent to it, for 0≤*k*≤5.^{ [22] }

Any point in the plane is on either zero, one or two tangent lines of a conic. A point on just one tangent line is on the conic. A point on no tangent line is said to be an **interior point** (or **inner point**) of the conic, while a point on two tangent lines is an **exterior point** (or **outer point**).

All the conic sections share a *reflection property* that can be stated as: All mirrors in the shape of a non-degenerate conic section reflect light coming from or going toward one focus toward or away from the other focus. In the case of the parabola, the second focus needs to be thought of as infinitely far away, so that the light rays going toward or coming from the second focus are parallel.^{ [23] }^{ [24] }

Pascal's theorem concerns the collinearity of three points that are constructed from a set of six points on any non-degenerate conic. The theorem also holds for degenerate conics consisting of two lines, but in that case it is known as Pappus's theorem.

Non-degenerate conic sections are always "smooth". This is important for many applications, such as aerodynamics, where a smooth surface is required to ensure laminar flow and to prevent turbulence.

It is believed that the first definition of a conic section was given by Menaechmus (died 320 BCE) as part of his solution of the Delian problem (Duplicating the cube).^{ [25] }^{ [26] } His work did not survive, not even the names he used for these curves, and is only known through secondary accounts.^{ [27] } The definition used at that time differs from the one commonly used today. Cones were constructed by rotating a right triangle about one of its legs so the hypotenuse generates the surface of the cone (such a line is called a generatrix). Three types of cones were determined by their vertex angles (measured by twice the angle formed by the hypotenuse and the leg being rotated about in the right triangle). The conic section was then determined by intersecting one of these cones with a plane drawn perpendicular to a generatrix. The type of the conic is determined by the type of cone, that is, by the angle formed at the vertex of the cone: If the angle is acute then the conic is an ellipse; if the angle is right then the conic is a parabola; and if the angle is obtuse then the conic is a hyperbola (but only one branch of the curve).^{ [28] }

Euclid (fl. 300 BCE) is said to have written four books on conics but these were lost as well.^{ [29] } Archimedes (died c. 212 BCE) is known to have studied conics, having determined the area bounded by a parabola and a chord in *Quadrature of the Parabola*. His main interest was in terms of measuring areas and volumes of figures related to the conics and part of this work survives in his book on the solids of revolution of conics, *On Conoids and Spheroids*.^{ [30] }

The greatest progress in the study of conics by the ancient Greeks is due to Apollonius of Perga (died c. 190 BCE), whose eight-volume *Conic Sections* or *Conics* summarized and greatly extended existing knowledge. Apollonius's study of the properties of these curves made it possible to show that any plane cutting a fixed double cone (two napped), regardless of its angle, will produce a conic according to the earlier definition, leading to the definition commonly used today. Circles, not constructible by the earlier method, are also obtainable in this way. This may account for why Apollonius considered circles a fourth type of conic section, a distinction that is no longer made. Apollonius used the names *ellipse*, *parabola* and *hyperbola* for these curves, borrowing the terminology from earlier Pythagorean work on areas.^{ [31] }

Pappus of Alexandria (died c. 350 CE) is credited with expounding on the importance of the concept of a conic's focus, and detailing the related concept of a directrix, including the case of the parabola (which is lacking in Apollonius's known works).^{ [32] }

An instrument for drawing conic sections was first described in 1000 CE by the Islamic mathematician Al-Kuhi.^{ [33] }^{ [34] }

Apollonius's work was translated into Arabic, and much of his work only survives through the Arabic version. Persians found applications of the theory, most notably the Persian^{ [35] } mathematician and poet Omar Khayyám, who used conic sections to solve algebraic equations of no higher a degree than three.^{ [36] }^{ [37] }

Johannes Kepler extended the theory of conics through the "principle of continuity", a precursor to the concept of limits. Kepler first used the term *foci* in 1604.^{ [38] }

Girard Desargues and Blaise Pascal developed a theory of conics using an early form of projective geometry and this helped to provide impetus for the study of this new field. In particular, Pascal discovered a theorem known as the hexagrammum mysticum from which many other properties of conics can be deduced.

René Descartes and Pierre Fermat both applied their newly discovered analytic geometry to the study of conics. This had the effect of reducing the geometrical problems of conics to problems in algebra. However, it was John Wallis in his 1655 treatise *Tractatus de sectionibus conicis* who first defined the conic sections as instances of equations of second degree.^{ [39] } Written earlier, but published later, Jan de Witt's *Elementa Curvarum Linearum* starts with Kepler's kinematic construction of the conics and then develops the algebraic equations. This work, which uses Fermat's methodology and Descartes' notation has been described as the first textbook on the subject.^{ [40] } De Witt invented the term *directrix*.^{ [40] }

Conic sections are important in astronomy: the orbits of two massive objects that interact according to Newton's law of universal gravitation are conic sections if their common center of mass is considered to be at rest. If they are bound together, they will both trace out ellipses; if they are moving apart, they will both follow parabolas or hyperbolas. See two-body problem.

The reflective properties of the conic sections are used in the design of searchlights, radio-telescopes and some optical telescopes.^{ [41] } A searchlight uses a parabolic mirror as the reflector, with a bulb at the focus; and a similar construction is used for a parabolic microphone. The 4.2 meter Herschel optical telescope on La Palma, in the Canary islands, uses a primary parabolic mirror to reflect light towards a secondary hyperbolic mirror, which reflects it again to a focus behind the first mirror.

The conic sections have some very similar properties in the Euclidean plane and the reasons for this become clearer when the conics are viewed from the perspective of a larger geometry. The Euclidean plane may be embedded in the real projective plane and the conics may be considered as objects in this projective geometry. One way to do this is to introduce homogeneous coordinates and define a conic to be the set of points whose coordinates satisfy an irreducible quadratic equation in three variables (or equivalently, the zeros of an irreducible quadratic form). More technically, the set of points that are zeros of a quadratic form (in any number of variables) is called a quadric, and the irreducible quadrics in a two dimensional projective space (that is, having three variables) are traditionally called conics.

The Euclidean plane *R*^{2} is embedded in the real projective plane by adjoining a line at infinity (and its corresponding points at infinity) so that all the lines of a parallel class meet on this line. On the other hand, starting with the real projective plane, a Euclidean plane is obtained by distinguishing some line as the line at infinity and removing it and all its points.

In a projective space over any division ring, but in particular over either the real or complex numbers, all non-degenerate conics are equivalent, and thus in projective geometry one simply speaks of "a conic" without specifying a type. That is, there is a projective transformation that will map any non-degenerate conic to any other non-degenerate conic.^{ [42] }

The three types of conic sections will reappear in the affine plane obtained by choosing a line of the projective space to be the line at infinity. The three types are then determined by how this line at infinity intersects the conic in the projective space. In the corresponding affine space, one obtains an ellipse if the conic does not intersect the line at infinity, a parabola if the conic intersects the line at infinity in one double point corresponding to the axis, and a hyperbola if the conic intersects the line at infinity in two points corresponding to the asymptotes.^{ [43] }

In homogeneous coordinates a conic section can be represented as:

Or in matrix notation

The 3 × 3 matrix above is called *the matrix of the conic section*.

Some authors prefer to write the general homogeneous equation as

(or some variation of this) so that the matrix of the conic section has the simpler form,

but this notation is not used in this article.^{ [44] }

If the determinant of the matrix of the conic section is zero, the conic section is degenerate.

As multiplying all six coefficients by the same non-zero scalar yields an equation with the same set of zeros, one can consider conics, represented by (*A*, *B*, *C*, *D*, *E*, *F*) as points in the five-dimensional projective space

Metrical concepts of Euclidean geometry (concepts concerned with measuring lengths and angles) can not be immediately extended to the real projective plane.^{ [45] } They must be redefined (and generalized) in this new geometry. This can be done for arbitrary projective planes, but to obtain the real projective plane as the extended Euclidean plane, some specific choices have to be made.^{ [46] }

Fix an arbitrary line in a projective plane that shall be referred to as the **absolute line**. Select two distinct points on the absolute line and refer to them as **absolute points**. Several metrical concepts can be defined with reference to these choices. For instance, given a line containing the points A and B, the **midpoint** of line segment AB is defined as the point C which is the projective harmonic conjugate of the point of intersection of AB and the absolute line, with respect to A and B.

A conic in a projective plane that contains the two absolute points is called a **circle**. Since five points determine a conic, a circle (which may be degenerate) is determined by three points. To obtain the extended Euclidean plane, the absolute line is chosen to be the line at infinity of the Euclidean plane and the absolute points are two special points on that line called the circular points at infinity. Lines containing two points with real coordinates do not pass through the circular points at infinity, so in the Euclidean plane a circle, under this definition, is determined by three points that are not collinear.^{ [47] }

It has been mentioned that circles in the Euclidean plane can not be defined by the focus-directrix property. However, if one were to consider the line at infinity as the directrix, then by taking the eccentricity to be *e* = 0 a circle will have the focus-directrix property, but it is still not defined by that property.^{ [48] } One must be careful in this situation to correctly use the definition of eccentricity as the ratio of the distance of a point on the circle to the focus (length of a radius) to the distance of that point to the directrix (this distance is infinite) which gives the limiting value of zero.

A synthetic (coordinate-free) approach to defining the conic sections in a projective plane was given by Jakob Steiner in 1867.

- Given two pencils of lines at two points (all lines containing and resp.) and a projective but not perspective mapping of onto . Then the intersection points of corresponding lines form a non-degenerate projective conic section.
^{ [49] }^{ [50] }^{ [51] }^{ [52] }

A *perspective* mapping of a pencil onto a pencil is a bijection (1-1 correspondence) such that corresponding lines intersect on a fixed line , which is called the *axis* of the perspectivity .

A *projective* mapping is a finite sequence of perspective mappings.

As a projective mapping in a projective plane over a field (pappian plane) is uniquely determined by prescribing the images of three lines,^{ [53] } for the Steiner generation of a conic section, besides two points only the images of 3 lines have to be given. These 5 items (2 points, 3 lines) uniquely determine the conic section.

By the Principle of Duality in a projective plane, the dual of each point is a line, and the dual of a locus of points (a set of points satisfying some condition) is called an *envelope* of lines. Using Steiner's definition of a conic (this locus of points will now be referred to as a *point conic*) as the meet of corresponding rays of two related pencils, it is easy to dualize and obtain the corresponding envelope consisting of the joins of corresponding points of two related ranges (points on a line) on different bases (the lines the points are on). Such an envelope is called a **line conic** (or **dual conic**).

In the real projective plane, a point conic has the property that every line meets it in two points (which may coincide, or may be complex) and any set of points with this property is a point conic. It follows dually that a line conic has two of its lines through every point and any envelope of lines with this property is a line conic. At every point of a point conic there is a unique tangent line, and dually, on every line of a line conic there is a unique point called a *point of contact*. An important theorem states that the tangent lines of a point conic form a line conic, and dually, the points of contact of a line conic form a point conic.^{ [54] }

Karl Georg Christian von Staudt defined a conic as the point set given by all the absolute points of a polarity that has absolute points. Von Staudt introduced this definition in *Geometrie der Lage* (1847) as part of his attempt to remove all metrical concepts from projective geometry.

A **polarity**, π, of a projective plane, *P*, is an involutory (i.e., of order two) bijection between the points and the lines of *P* that preserves the incidence relation. Thus, a polarity relates a point *Q* with a line *q* and, following Gergonne, *q* is called the **polar** of *Q* and *Q* the **pole** of *q*.^{ [55] } An **absolute point** (**line**) of a polarity is one which is incident with its polar (pole).^{ [56] }

A von Staudt conic in the real projective plane is equivalent to a Steiner conic.^{ [57] }

No continuous arc of a conic can be constructed with straightedge and compass. However, there are several straightedge-and-compass constructions for any number of individual points on an arc.

One of them is based on the converse of Pascal's theorem, namely, *if the points of intersection of opposite sides of a hexagon are collinear, then the six vertices lie on a conic.* Specifically, given five points, *A*, *B*, *C*, *D*, *E* and a line passing through E, say *EG*, \a point F that lies on this line and is on the conic determined by the five points can be constructed. Let *AB* meet *DE* in L, *BC* meet *EG* in M and let *CD* meet *LM* at N. Then *AN* meets *EG* at the required point F.^{ [58] } By varying the line through E,as many additional points on the conic as desired can be constructed.

Another method, based on Steiner's construction and which is useful in engineering applications, is the **parallelogram method**, where a conic is constructed point by point by means of connecting certain equally spaced points on a horizontal line and a vertical line.^{ [59] } Specifically, to construct the ellipse with equation *x*^{2}/*a*^{2} + *y*^{2}/*b*^{2} = 1, first construct the rectangle *ABCD* with vertices *A*(*a*, 0), *B*(*a*, 2*b*), *C*(−*a*, 2*b*) and *D*(−*a*, 0). Divide the side *BC* into n equal segments and use parallel projection, with respect to the diagonal *AC*, to form equal segments on side *AB* (the lengths of these segments will be *b*/*a* times the length of the segments on *BC*). On the side *BC* label the left-hand endpoints of the segments with *A*_{1} to *A*_{n} starting at B and going towards C. On the side *AB* label the upper endpoints *D*_{1} to *D*_{n} starting at A and going towards B. The points of intersection, *AA*_{i} ∩ *DD*_{i} for 1 ≤ *i* ≤ *n* will be points of the ellipse between A and *P*(0, *b*). The labeling associates the lines of the pencil through A with the lines of the pencil through D projectively but not perspectively. The sought for conic is obtained by this construction since three points *A*, *D* and P and two tangents (the vertical lines at A and D) uniquely determine the conic. If another diameter (and its conjugate diameter) are used instead of the major and minor axes of the ellipse, a parallelogram that is not a rectangle is used in the construction, giving the name of the method. The association of lines of the pencils can be extended to obtain other points on the ellipse. The constructions for hyperbolas^{ [60] } and parabolas^{ [61] } are similar.

Yet another general method uses the polarity property to construct the tangent envelope of a conic (a line conic).^{ [62] }

In the complex plane **C ^{2}**, ellipses and hyperbolas are not distinct: one may consider a hyperbola as an ellipse with an imaginary axis length. For example, the ellipse becomes a hyperbola under the substitution geometrically a complex rotation, yielding . Thus there is a 2-way classification: ellipse/hyperbola and parabola. Extending the curves to the complex projective plane, this corresponds to intersecting the line at infinity in either 2 distinct points (corresponding to two asymptotes) or in 1 double point (corresponding to the axis of a parabola); thus the real hyperbola is a more suggestive real image for the complex ellipse/hyperbola, as it also has 2 (real) intersections with the line at infinity.

Further unification occurs in the complex projective plane **CP ^{2}**: the non-degenerate conics cannot be distinguished from one another, since any can be taken to any other by a projective linear transformation.

It can be proven that in **CP ^{2}**, two conic sections have four points in common (if one accounts for multiplicity), so there are between 1 and 4 intersection points. The intersection possibilities are: four distinct points, two singular points and one double point, two double points, one singular point and one with multiplicity 3, one point with multiplicity 4. If any intersection point has multiplicity > 1, the two curves are said to be tangent. If there is only one intersection point, which has multiplicity 4, the two curves are said to be osculating.

Furthermore, each straight line intersects each conic section twice. If the intersection point is double, the line is a tangent line. Intersecting with the line at infinity, each conic section has two points at infinity. If these points are real, the curve is a hyperbola; if they are imaginary conjugates, it is an ellipse; if there is only one double point, it is a parabola. If the points at infinity are (1,i,0) and (1,-i,0), the conic section is a circle (see circular points at infinity). If a conic section has one real and one imaginary point at infinity, or two imaginary points that are not conjugates, then it is not a real conic section: its coefficients cannot be real.

What should be considered as a **degenerate case** of a conic depends on the definition being used and the geometric setting for the conic section. There are some authors who define a conic as a two-dimensional nondegenerate quadric. With this terminology there are no degenerate conics (only degenerate quadrics), but we shall use the more traditional terminology and avoid that definition.

In the Euclidean plane, using the geometric definition, a degenerate case arises when the cutting plane passes through the apex of the cone. The degenerate conic is either: a point, when the plane intersects the cone only at the apex; a straight line, when the plane is tangent to the cone (it contains exactly one generator of the cone); or a pair of intersecting lines (two generators of the cone).^{ [64] } These correspond respectively to the limiting forms of an ellipse, parabola, and a hyperbola.

If a conic in the Euclidean plane is being defined by the zeros of a quadratic equation (that is, as a quadric), then the degenerate conics are: the empty set, a point, or a pair of lines which may be parallel, intersect at a point, or coincide. The empty set case may correspond either to a pair of complex conjugate parallel lines such as with the equation or to an *imaginary ellipse*, such as with the equation An imaginary ellipse does not satisfy the general definition of a degeneracy, and is thus not normally considered as degenerated. The two lines case occurs when the quadratic expression factors into two linear factors, the zeros of each giving a line. In the case that the factors are the same, the corresponding lines coincide and we refer to the line as a *double* line (a line with multiplicity 2) and this is the previous case of a tangent cutting plane.

In the real projective plane, since parallel lines meet at a point on the line at infinity, the parallel line case of the Euclidean plane can be viewed as intersecting lines. However, as the point of intersection is the apex of the cone, the cone itself degenerates to a cylinder, i.e. with the apex at infinity. Other sections in this case are called *cylindric sections*.^{ [65] } The non-degenerate cylindrical sections are ellipses (or circles).

When viewed from the perspective of the complex projective plane, the degenerate cases of a real quadric (i.e., the quadratic equation has real coefficients) can all be considered as a pair of lines, possibly coinciding. The empty set may be the line at infinity considered as a double line, a (real) point is the intersection of two complex conjugate lines and the other cases as previously mentioned.

To distinguish the degenerate cases from the non-degenerate cases (including the empty set with the latter) using matrix notation, let *β* be the determinant of the 3×3 matrix of the conic section—that is, *β * = (*AC* − *B*^{2}/4)*F* + *BED* − *CD*^{2} − *AE*^{2}/4; and let *α* = *B*^{2} − 4*AC* be the discriminant. Then the conic section is non-degenerate if and only if *β* ≠ 0. If *β *= 0 we have a point when *α* < 0, two parallel lines (possibly coinciding) when *α* = 0, or two intersecting lines when *α* > 0.^{ [66] }

A (non-degenerate) conic is completely determined by five points in general position (no three collinear) in a plane and the system of conics which pass through a fixed set of four points (again in a plane and no three collinear) is called a **pencil of conics**.^{ [67] } The four common points are called the *base points* of the pencil. Through any point other than a base point, there passes a single conic of the pencil. This concept generalizes a pencil of circles.

In a projective plane defined over an algebraically closed field any two conics meet in four points (counted with multiplicity) and so, determine the pencil of conics based on these four points. Furthermore, the four base points determine three line pairs (degenerate conics through the base points, each line of the pair containing exactly two base points) and so each pencil of conics will contain at most three degenerate conics.^{ [68] }

A pencil of conics can represented algebraically in the following way. Let *C*_{1} and *C*_{2} be two distinct conics in a projective plane defined over an algebraically closed field *K*. For every pair *λ*, *μ* of elements of *K*, not both zero, the expression:

represents a conic in the pencil determined by *C*_{1} and *C*_{2}. This symbolic representation can be made concrete with a slight abuse of notation (using the same notation to denote the object as well as the equation defining the object.) Thinking of *C*_{1}, say, as a ternary quadratic form, then *C*_{1} = 0 is the equation of the "conic *C*_{1}". Another concrete realization would be obtained by thinking of *C*_{1} as the 3×3 symmetric matrix which represents it. If *C*_{1} and *C*_{2} have such concrete realizations then every member of the above pencil will as well. Since the setting uses homogeneous coordinates in a projective plane, two concrete representations (either equations or matrices) give the same conic if they differ by a non-zero multiplicative constant.

The solutions to a system of two second degree equations in two variables may be viewed as the coordinates of the points of intersection of two generic conic sections. In particular two conics may possess none, two or four possibly coincident intersection points. An efficient method of locating these solutions exploits the homogeneous matrix representation of conic sections, i.e. a 3x3 symmetric matrix which depends on six parameters.

The procedure to locate the intersection points follows these steps, where the conics are represented by matrices:

- given the two conics and , consider the pencil of conics given by their linear combination
- identify the homogeneous parameters which correspond to the degenerate conic of the pencil. This can be done by imposing the condition that and solving for and . These turn out to be the solutions of a third degree equation.
- given the degenerate conic , identify the two, possibly coincident, lines constituting it.
- intersect each identified line with either one of the two original conics; this step can be done efficiently using the dual conic representation of
- the points of intersection will represent the solutions to the initial equation system.

Conics may be defined over other fields (that is, in other pappian geometries). However, some care must be used when the field has characteristic 2, as some formulas can not be used. For example, the matrix representations used above require division by 2.

A generalization of a non-degenerate conic in a projective plane is an oval. An oval is a point set that has the following properties, which are held by conics: 1) any line intersects an oval in none, one or two points, 2) at any point of the oval there exists a unique tangent line.

Generalizing the focus properties of conics to the case where there are more than two foci produces sets called generalized conics.

The classification into elliptic, parabolic, and hyperbolic is pervasive in mathematics, and often divides a field into sharply distinct subfields. The classification mostly arises due to the presence of a quadratic form (in two variables this corresponds to the associated discriminant), but can also correspond to eccentricity.

Quadratic form classifications:

- Quadratic forms
- Quadratic forms over the reals are classified by Sylvester's law of inertia, namely by their positive index, zero index, and negative index: a quadratic form in
*n*variables can be converted to a diagonal form, as where the number of +1 coefficients,*k,*is the positive index, the number of −1 coefficients,*ℓ*, is the negative index, and the remaining variables are the zero index*m,*so In two variables the non-zero quadratic forms are classified as:- – positive-definite (the negative is also included), corresponding to ellipses,
- – degenerate, corresponding to parabolas, and
- – indefinite, corresponding to hyperbolas.

- In two variables quadratic forms are classified by discriminant, analogously to conics, but in higher dimensions the more useful classification is as
*definite,*(all positive or all negative),*degenerate,*(some zeros), or*indefinite*(mix of positive and negative but no zeros). This classification underlies many that follow. - Curvature
- The Gaussian curvature of a surface describes the infinitesimal geometry, and may at each point be either positive – elliptic geometry, zero – Euclidean geometry (flat, parabola), or negative – hyperbolic geometry; infinitesimally, to second order the surface looks like the graph of (or 0), or . Indeed, by the uniformization theorem every surface can be taken to be globally (at every point) positively curved, flat, or negatively curved. In higher dimensions the Riemann curvature tensor is a more complicated object, but manifolds with constant sectional curvature are interesting objects of study, and have strikingly different properties, as discussed at sectional curvature.
- Second order PDEs
- Partial differential equations (PDEs) of second order are classified at each point as elliptic, parabolic, or hyperbolic, accordingly as their second order terms correspond to an elliptic, parabolic, or hyperbolic quadratic form. The behavior and theory of these different types of PDEs are strikingly different – representative examples is that the Poisson equation is elliptic, the heat equation is parabolic, and the wave equation is hyperbolic.

Eccentricity classifications include:

- Möbius transformations
- Real Möbius transformations (elements of PSL
_{2}(**R**) or its 2-fold cover, SL_{2}(**R**)) are classified as elliptic, parabolic, or hyperbolic accordingly as their half-trace is or mirroring the classification by eccentricity. - Variance-to-mean ratio
- The variance-to-mean ratio classifies several important families of discrete probability distributions: the constant distribution as circular (eccentricity 0), binomial distributions as elliptical, Poisson distributions as parabolic, and negative binomial distributions as hyperbolic. This is elaborated at cumulants of some discrete probability distributions.

- Circumconic and inconic
- Conic Sections Rebellion, protests by Yale university students
- Director circle
- Elliptic coordinate system
- Equidistant set
- Nine-point conic
- Parabolic coordinates
- Quadratic function

- ↑ Eves 1963 , p. 319
- ↑ Brannan, Esplen & Gray 1999 , p. 13
- ↑ Thomas & Finney 1979 , p. 434
- ↑ Brannan, Esplen & Gray 1999 , p. 19; Kendig 2005 , pp. 86, 141
- ↑ Brannan, Esplen & Gray 1999 , pp. 13–16
- ↑ Brannan, Esplen & Gray 1999 , pp. 11–16
- ↑ Protter & Morrey 1970 , pp. 314–328, 585–589
- ↑ Protter & Morrey 1970 , pp. 290–314
- ↑ Wilson & Tracey 1925 , p. 130
- ↑ the empty set is included as a degenerate conic since it may arise as a solution of this equation
- ↑ Protter & Morrey 1970 , p. 316
- ↑ Brannan, Esplen & Gray 1999 , p. 30
- ↑ Fanchi, John R. (2006),
*Math refresher for scientists and engineers*, John Wiley and Sons, pp. 44–45, ISBN 0-471-75715-2 , Section 3.2, page 45 - 1 2 Protter & Morrey 1970 , p. 326
- ↑ Wilson & Tracey 1925 , p. 153
- ↑ Pettofrezzo, Anthony,
*Matrices and Transformations*, Dover Publ., 1966, p. 110. - 1 2 3 Spain, Barry,
*Analytical Conics*, Dover, 2007 (originally published 1957 by Pergamon Press). - ↑ Ayoub, Ayoub B., "The eccentricity of a conic section,"
*The College Mathematics Journal*34(2), March 2003, 116–121. - ↑ Ayoub, A. B., "The central conic sections revisited",
*Mathematics Magazine*66(5), 1993, 322–325. - ↑ Brannan, Esplen & Gray 1999 , p. 17
- ↑ Whitworth, William Allen.
*Trilinear Coordinates and Other Methods of Modern Analytical Geometry of Two Dimensions*, Forgotten Books, 2012 (orig. Deighton, Bell, and Co., 1866), p. 203 http://www.forgottenbooks.com/search?q=Trilinear+coordinates&t=books - ↑ Paris Pamfilos, "A gallery of conics by five elements",
*Forum Geometricorum*14, 2014, 295–348. http://forumgeom.fau.edu/FG2014volume14/FG201431.pdf - ↑ Brannan, Esplen & Gray 1999 , p. 28
- ↑ Downs 2003 , pp. 36ff.
- ↑ According to Plutarch this solution was rejected by Plato on the grounds that it could not be achieved using only straightedge and compass, however this interpretation of Plutarch's statement has come under criticism.Boyer 2004 , p.14, footnote 14
- ↑ Boyer 2004 , pp. 17–18
- ↑ Boyer 2004 , p. 18
- ↑ Katz 1998 , p. 117
- ↑ Heath, T.L.,
*The Thirteen Books of Euclid's Elements*, Vol. I, Dover, 1956, pg.16 - ↑ Eves 1963 , p. 28
- ↑ Eves 1963 , p. 30
- ↑ Boyer 2004 , p. 36
- ↑ Stillwell, John (2010).
*Mathematics and its history*(3rd ed.). New York: Springer. p. 30. ISBN 1-4419-6052-X. - ↑ "Apollonius of Perga Conics Books One to Seven" (PDF). Retrieved 10 June 2011.
- ↑ Turner, Howard R. (1997).
*Science in medieval Islam: an illustrated introduction*. University of Texas Press. p. 53. ISBN 0-292-78149-0., Chapter , p. 53 - ↑ Boyer, C. B., & Merzbach, U. C.,
*A History of Mathematics*(Hoboken: John Wiley & Sons, Inc., 1968), p. 219. - ↑ Van der Waerden, B. L.,
*Geometry and Algebra in Ancient Civilizations*(Berlin/Heidelberg: Springer Verlag, 1983), p. 73. - ↑ Katz 1998 , p. 126
- ↑ Boyer 2004 , p. 110
- 1 2 Boyer 2004 , p. 114
- ↑ Brannan, Esplen & Gray 1999 , p. 27
- ↑ Artzy 2008 , p. 158, Thm 3-5.1
- ↑ Artzy 2008 , p. 159
- ↑ This form of the equation does not generalize to fields of characteristic two (see below)
- ↑ Consider finding the midpoint of a line segment with one endpoint on the line at infinity.
- ↑ Faulkner 1952 , p. 71
- ↑ Faulkner 1952 , p. 72
- ↑ Eves 1963 , p. 320
- ↑ Coxeter 1993, p. 80
- ↑ Hartmann , p. 38
- ↑ Merserve 1983 , p. 65
- ↑
*Jacob Steiner’s Vorlesungen über synthetische Geometrie*, B. G. Teubner, Leipzig 1867 (from Google Books: (German) Part II follows Part I) Part II, pg. 96 - ↑ Hartmann , p. 19
- ↑ Faulkner 1952 , pp. 48–49
- ↑ Coxeter 1964 , p. 60
- ↑ Coxeter and several other authors use the term
*self-conjugate*instead of absolute. - ↑ Coxeter 1964 , p. 80
- ↑ Faulkner 1952 , pp. 52–53
- ↑ Downs 2003 , p. 5
- ↑ Downs 2003 , p. 14
- ↑ Downs 2003 , p. 19
- ↑ Akopyan & Zaslavsky 2007 , p. 70
- ↑ Wilczynski, E. J. (1916), "Some remarks on the historical development and the future prospects of the differential geometry of plane curves",
*Bull. Amer. Math. Soc.*,**22**: 317–329, doi:10.1090/s0002-9904-1916-02785-6 . - ↑ Brannan, Esplen & Gray 1999 , p. 6
- ↑ "MathWorld: Cylindric section".
- ↑ Lawrence, J. Dennis (1972),
*A Catalog of Special Plane Curves*, Dover, p. 63, ISBN 0-486-60288-5 - ↑ Faulkner 1952 , pg. 64
- ↑ Samuel 1988 , pg. 50

In elementary geometry, the property of being **perpendicular** (**perpendicularity**) is the relationship between two lines which meet at a right angle. The property extends to other related geometric objects.

In mathematics, a **quadric** or **quadric surface**, is a generalization of conic sections. It is a hypersurface in a (*D* + 1)-dimensional space, and it is defined as the zero set of an irreducible polynomial of degree two in *D* + 1 variables. When the defining polynomial is not absolutely irreducible, the zero set is generally not considered a quadric, although it is often called a *degenerate quadric* or a *reducible quadric*.

In algebra, a **quadratic function**, a **quadratic polynomial**, a **polynomial of degree 2**, or simply a **quadratic**, is a polynomial function with one or more variables in which the highest-degree term is of the second degree. For example, a quadratic function in three variables *x*, *y,* and *z* contains exclusively terms *x*^{2}, *y*^{2}, *z*^{2}, *xy*, *xz*, *yz*, *x*, *y*, *z*, and a constant:

In mathematics, the **matrix representation of conic sections** permits the tools of linear algebra to be used in the study of conic sections. It provides easy ways to calculate a conic section's axis, vertices, tangents and the pole and polar relationship between points and lines of the plane determined by the conic. The technique does not require putting the equation of a conic section into a standard form, thus making it easier to investigate those conic sections whose axes are not parallel to the coordinate system.

In geometry, a **degenerate conic** is a conic that fails to be an irreducible curve. This means that the defining equation is factorable over the complex numbers as the product of two linear polynomials.

In geometry, the **Dandelin spheres** are one or two spheres that are tangent both to a plane and to a cone that intersects the plane. The intersection of the cone and the plane is a conic section, and the point at which either sphere touches the plane is a focus of the conic section, so the Dandelin spheres are also sometimes called **focal spheres**.

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.

A **cone** is a three-dimensional geometric shape that tapers smoothly from a flat base to a point called the apex or vertex.

A **cylinder** has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. It is the idealized version of a solid physical tin can having lids on top and bottom.

In geometry, the **pole** and **polar** are respectively a point and a line that have a unique reciprocal relationship with respect to a given conic section.

In geometry, the **director circle** of an ellipse or hyperbola is a circle consisting of all points where two perpendicular tangent lines to the ellipse or hyperbola cross each other.

In geometry, the **unit hyperbola** is the set of points (*x,y*) in the Cartesian plane that satisfy the implicit equation In the study of indefinite orthogonal groups, the unit hyperbola forms the basis for an *alternative radial length*

In the geometry of curves, an **orthoptic** is the set of points for which two tangents of a given curve meet at a right angle.

The **Steiner conic** or more precisely **Steiner's generation of a conic**, named after the Swiss mathematician Jakob Steiner, is an alternative method to define a non-degenerate projective conic section in a projective plane over a field.

In mathematics, a **generalized conic** is a geometrical object defined by a property which is a generalization of some defining property of the classical conic. For example, in elementary geometry, an ellipse can be defined as the locus of a point which moves in a plane such that the sum of its distances from two fixed points – the foci – in the plane is a constant. The curve obtained when the set of two fixed points is replaced by an arbitrary, but fixed, finite set of points in the plane is called an *n*–ellipse and can be thought of as a generalized ellipse. Since an ellipse is the equidistant set of two circles, the equidistant set of two arbitrary sets of points in a plane can be viewed as a generalized conic. In rectangular Cartesian coordinates, the equation *y* = *x*^{2} represents a parabola. The generalized equation *y* = *x*^{r}, for *r* ≠ 0 and *r* ≠ 1, can be treated as defining a generalized parabola. The idea of generalized conic has found applications in approximation theory and optimization theory.

In geometry, two conic sections are called **confocal**, if they have the same foci. Because ellipses and hyperbolas possess two foci, there are **confocal ellipses**, **confocal hyperbolas** and confocal mixtures of ellipses and hyperbolas. In the mixture of confocal ellipses and hyperbolas, any ellipse intersects any hyperbola orthogonally. Parabolas possess only one focus, so, by convention, **confocal parabolas** have the same focus *and* the same axis of symmetry. Consequently, any point not on the axis of symmetry lies on two confocal parabolas which intersect orthogonally.

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*Linear Geometry*, Dover, ISBN 978-0-486-46627-9 - Boyer, Carl B. (2004) [1956],
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*Geometry*, Cambridge University Press, ISBN 978-0-521-59787-6 - Coxeter, H.S.M. (1964),
*Projective Geometry*, Blaisdell - Coxeter, H.S.M. (1993),
*The Real Projective Plane*, Springer Science & Business Media - Downs, J.W. (2003) [1993],
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*Planar Circle Geometries, an Introduction to Moebius-, Laguerre- and Minkowski Planes*(PDF), retrieved 20 September 2014 (PDF; 891 kB). - Katz, Victor J. (1998),
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*Fundamental Concepts of Geometry*, Dover, ISBN 0-486-63415-9 - Protter, Murray H.; Morrey, Jr., Charles B. (1970),
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*Calculus and Analytic Geometry*(fifth ed.), Addison-Wesley, p. 434, ISBN 0-201-07540-7 - Wilson, W.A.; Tracey, J.I. (1925),
*Analytic Geometry*(Revised ed.), D.C. Heath and Company

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- Conic section (Geometry) at the
*Encyclopædia Britannica* - Can You Really Derive Conic Formulae from a Cone? archive 2007-07-15 Gary S. Stoudt (Indiana University of Pennsylvania
- Conic sections at Special plane curves.
- Weisstein, Eric W. "Conic Section".
*MathWorld*. - Occurrence of the conics. Conics in nature and elsewhere.
- See Conic Sections at cut-the-knot for a sharp proof that any finite conic section is an ellipse and Xah Lee for a similar treatment of other conics.
- Eight Point Conic at Dynamic Geometry Sketches
- Second-order implicit equation locus An interactive Java conics grapher; uses a general second-order implicit equation.

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