# Hyperbola

In mathematics, a **hyperbola** (; pl. **hyperbolas** or **hyperbolae** ; adj. **hyperbolic** ) 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. (The other conic sections are the parabola and the ellipse. A circle is a special case of an ellipse.) 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.

Hyperbolas share many of the ellipses' analytical properties such as eccentricity, focus, and directrix. Typically the correspondence can be made with nothing more than a change of sign in some term. Many other mathematical objects have their origin in the hyperbola, such as hyperbolic paraboloids (saddle surfaces), hyperboloids ("wastebaskets"), hyperbolic geometry (Lobachevsky's celebrated non-Euclidean geometry), hyperbolic functions (sinh, cosh, tanh, etc.), and gyrovector spaces (a geometry proposed for use in both relativity and quantum mechanics which is not Euclidean).

The word "hyperbola" derives from the Greek ὑπερβολή, meaning "over-thrown" or "excessive", from which the English term hyperbole also derives. Hyperbolae were discovered by Menaechmus in his investigations of the problem of doubling the cube, but were then called sections of obtuse cones.^{[3]} The term hyperbola is believed to have been coined by Apollonius of Perga (c. 262–c. 190 BC) in his definitive work on the conic sections, the *Conics*.^{[4]} The names of the other two general conic sections, the ellipse and the parabola, derive from the corresponding Greek words for "deficient" and "applied"; all three names are borrowed from earlier Pythagorean terminology which referred to a comparison of the side of rectangles of fixed area with a given line segment. The rectangle could be "applied" to the segment (meaning, have an equal length), be shorter than the segment or exceed the segment.^{[5]}

A hyperbola can be defined geometrically as a set of points (locus of points) in the Euclidean plane:

The *inverse statement* is also true and can be used to define a hyperbola (in a manner similar to the definition of a parabola):

The definition of a hyperbola by its foci and its circular directrices (see above) can be used for drawing an arc of it with help of pins, a string and a ruler:^{[10]}

The following method to construct single points of a hyperbola relies on the :

The following properties of a hyperbola are easily proven using the representation of a hyperbola introduced in this section.

This property provides a way to construct the tangent at a point on the hyperbola.

This property of a hyperbola is an affine version of the 3-point-degeneration of Pascal's theorem.^{[13]}

This property provides a possibility to construct points of a hyperbola if the asymptotes and one point are given.

This property of a hyperbola is an affine version of the 4-point-degeneration of Pascal's theorem.^{[14]}

The reciprocation of a circle *B* in a circle *C* always yields a conic section such as a hyperbola. The process of "reciprocation in a circle *C*" consists of replacing every line and point in a geometrical figure with their corresponding pole and polar, respectively. The *pole* of a line is the inversion of its closest point to the circle *C*, whereas the polar of a point is the converse, namely, a line whose closest point to *C* is the inversion of the point.

The eccentricity of the conic section obtained by reciprocation is the ratio of the distances between the two circles' centers to the radius *r* of reciprocation circle *C*. If **B** and **C** represent the points at the centers of the corresponding circles, then

Since the eccentricity of a hyperbola is always greater than one, the center **B** must lie outside of the reciprocating circle *C*.

This definition implies that the hyperbola is both the locus of the poles of the tangent lines to the circle *B*, as well as the envelope of the polar lines of the points on *B*. Conversely, the circle *B* is the envelope of polars of points on the hyperbola, and the locus of poles of tangent lines to the hyperbola. Two tangent lines to *B* have no (finite) poles because they pass through the center **C** of the reciprocation circle *C*; the polars of the corresponding tangent points on *B* are the asymptotes of the hyperbola. The two branches of the hyperbola correspond to the two parts of the circle *B* that are separated by these tangent points.

A hyperbola can also be defined as a second-degree equation in the Cartesian coordinates (*x*, *y*) in the plane,

provided that the constants *A*_{xx}, *A*_{xy}, *A*_{yy}, *B*_{x}, *B*_{y}, and *C* satisfy the determinant condition

This determinant is conventionally called the discriminant of the conic section.^{[15]}

A special case of a hyperbola—the *degenerate hyperbola* consisting of two intersecting lines—occurs when another determinant is zero:

This determinant Δ is sometimes called the discriminant of the conic section.^{[16]}

Given the above general parametrization of the hyperbola in Cartesian coordinates, the eccentricity can be found using the formula in .

The center (*x*_{c}, *y*_{c}) of the hyperbola may be determined from the formulae

In terms of new coordinates, *ξ* = *x* − *x*_{c} and *η* = *y* − *y*_{c}, the defining equation of the hyperbola can be written

The principal axes of the hyperbola make an angle *φ* with the positive *x*-axis that is given by

Rotating the coordinate axes so that the *x*-axis is aligned with the transverse axis brings the equation into its **canonical form**

For comparison, the corresponding equation for a degenerate hyperbola (consisting of two intersecting lines) is

The tangent line to a given point (*x*_{0}, *y*_{0}) on the hyperbola is defined by the equation

The normal line to the hyperbola at the same point is given by the equation

The normal line is perpendicular to the tangent line, and both pass through the same point (*x*_{0}, *y*_{0}).

If (*x*,*y*) is a point on the hyperbola the distance to the left focal point is

If Cartesian coordinates are introduced such that the origin is the center of the hyperbola and the *x*-axis is the major axis, then the hyperbola is called *east-west-opening* and

This equation is called the canonical form of a hyperbola, because any hyperbola, regardless of its orientation relative to the Cartesian axes and regardless of the location of its center, can be transformed to this form by a change of variables, giving a hyperbola that is congruent to the original (see below).

It follows from the equation that the hyperbola is *symmetric* with respect to both of the coordinate axes and hence symmetric with respect to the origin.

For a hyperbola in the above canonical form, the eccentricity is given by

Two hyperbolas are geometrically similar to each other – meaning that they have the same shape, so that one can be transformed into the other by rigid left and right movements, rotation, taking a mirror image, and scaling (magnification) – if and only if they have the same eccentricity.

*product of the distances from a point on the hyperbola to both the asymptotes*

*The product of the distances from a point on the hyperbola to the asymptotes along lines parallel to the asymptotes*

A particular tangent line distinguishes the hyperbola from the other conic sections.^{[22]} Let *f* be the distance from the vertex *V* (on both the hyperbola and its axis through the two foci) to the nearer focus. Then the distance, along a line perpendicular to that axis, from that focus to a point P on the hyperbola is greater than 2*f*. The tangent to the hyperbola at P intersects that axis at point Q at an angle ∠PQV of greater than 45°.

Further parametric representations are given in the section Parametric equations below.

With polar coordinates relative to the "canonical coordinate system" (see second diagram) one has that

Just as the trigonometric functions are defined in terms of the unit circle, so also the hyperbolic functions are defined in terms of the unit hyperbola, as shown in this diagram. In a unit circle, the angle (in radians) is equal to twice the area of the circular sector which that angle subtends. The analogous hyperbolic angle is likewise defined as twice the area of a hyperbolic sector.

Other hyperbolic functions are defined according to the hyperbolic cosine and hyperbolic sine, so for example

The midpoints of parallel chords of a hyperbola lie on a line through the center (see diagram).

The points of any chord may lie on different branches of the hyperbola.

If the chord degenerates into a *tangent*, then the touching point divides the line segment between the asymptotes in two halves.

The tangents may belong to points on different branches of the hyperbola.

Such a relation between points and lines generated by a conic is called **pole-polar relation** or just *polarity*. The pole is the point, the polar the line. See Pole and polar.

By calculation one checks the following properties of the pole-polar relation of the hyperbola:

The arc length of a hyperbola does not have an elementary expression. The upper half of a hyperbola can be parameterized as

Several other curves can be derived from the hyperbola by inversion, the so-called inverse curves of the hyperbola. If the center of inversion is chosen as the hyperbola's own center, the inverse curve is the lemniscate of Bernoulli; the lemniscate is also the envelope of circles centered on a rectangular hyperbola and passing through the origin. If the center of inversion is chosen at a focus or a vertex of the hyperbola, the resulting inverse curves are a limaçon or a strophoid, respectively.

A family of confocal hyperbolas is the basis of the system of elliptic coordinates in two dimensions. These hyperbolas are described by the equation

where the foci are located at a distance *c* from the origin on the *x*-axis, and where θ is the angle of the asymptotes with the *x*-axis. Every hyperbola in this family is orthogonal to every ellipse that shares the same foci. This orthogonality may be shown by a conformal map of the Cartesian coordinate system *w* = *z* + 1/*z*, where *z*= *x* + *iy* are the original Cartesian coordinates, and *w*=*u* + *iv* are those after the transformation.

Other orthogonal two-dimensional coordinate systems involving hyperbolas may be obtained by other conformal mappings. For example, the mapping *w* = *z*^{2} transforms the Cartesian coordinate system into two families of orthogonal hyperbolas.

Besides providing a uniform description of circles, ellipses, parabolas, and hyperbolas, conic sections can also be understood as a natural model of the geometry of perspective in the case where the scene being viewed consists of circles, or more generally an ellipse. The viewer is typically a camera or the human eye and the image of the scene a central projection onto an image plane, that is, all projection rays pass a fixed point *O*, the center. The **lens plane** is a plane parallel to the image plane at the lens *O*.

(Special positions where the circle plane contains point *O* are omitted.)

These results can be understood if one recognizes that the projection process can be seen in two steps: 1) circle c and point *O* generate a cone which is 2) cut by the image plane, in order to generate the image.

One sees a hyperbola whenever catching sight of a portion of a circle cut by one's lens plane. The inability to see very much of the arms of the visible branch, combined with the complete absence of the second branch, makes it virtually impossible for the human visual system to recognize the connection with hyperbolas.

Hyperbolas may be seen in many sundials. On any given day, the sun revolves in a circle on the celestial sphere, and its rays striking the point on a sundial traces out a cone of light. The intersection of this cone with the horizontal plane of the ground forms a conic section. At most populated latitudes and at most times of the year, this conic section is a hyperbola. In practical terms, the shadow of the tip of a pole traces out a hyperbola on the ground over the course of a day (this path is called the *declination line*). The shape of this hyperbola varies with the geographical latitude and with the time of the year, since those factors affect the cone of the sun's rays relative to the horizon. The collection of such hyperbolas for a whole year at a given location was called a *pelekinon* by the Greeks, since it resembles a double-bladed axe.

A hyperbola is the basis for solving multilateration problems, the task of locating a point from the differences in its distances to given points — or, equivalently, the difference in arrival times of synchronized signals between the point and the given points. Such problems are important in navigation, particularly on water; a ship can locate its position from the difference in arrival times of signals from a LORAN or GPS transmitters. Conversely, a homing beacon or any transmitter can be located by comparing the arrival times of its signals at two separate receiving stations; such techniques may be used to track objects and people. In particular, the set of possible positions of a point that has a distance difference of 2*a* from two given points is a hyperbola of vertex separation 2*a* whose foci are the two given points.

The path followed by any particle in the classical Kepler problem is a conic section. In particular, if the total energy *E* of the particle is greater than zero (that is, if the particle is unbound), the path of such a particle is a hyperbola. This property is useful in studying atomic and sub-atomic forces by scattering high-energy particles; for example, the Rutherford experiment demonstrated the existence of an atomic nucleus by examining the scattering of alpha particles from gold atoms. If the short-range nuclear interactions are ignored, the atomic nucleus and the alpha particle interact only by a repulsive Coulomb force, which satisfies the inverse square law requirement for a Kepler problem.

In portfolio theory, the locus of mean-variance efficient portfolios (called the efficient frontier) is the upper half of the east-opening branch of a hyperbola drawn with the portfolio return's standard deviation plotted horizontally and its expected value plotted vertically; according to this theory, all rational investors would choose a portfolio characterized by some point on this locus.

In biochemistry and pharmacology, the Hill equation and Hill-Langmuir equation respectively describe biological responses and the formation of protein–ligand complexes as functions of ligand concentration. They are both rectangular hyperbolae.