This article is about a geometric curve. For the term used in rhetoric, see
Hyperbole.
In
mathematics, a hyperbola (/haɪˈpɜːrbələ/^{
ⓘ}; pl. hyperbolas or hyperbolae/-liː/^{
ⓘ}; adj. hyperbolic/ˌhaɪpərˈbɒlɪk/^{
ⓘ}) is a type of
smoothcurve 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.
Besides being a conic section, a hyperbola can arise as the
locus of points whose difference of distances to two fixed
foci is constant, as a curve for each point of which the rays to two fixed foci are
reflections across the
tangent line at that point, or as the solution of certain bivariate
quadratic equations such as the
reciprocal relationship $xy=1.$^{
[1]} In practical applications, a hyperbola can arise as the path followed by the shadow of the tip of a
sundial's
gnomon, the shape of an
open orbit such as that of a celestial object exceeding the
escape velocity of the nearest gravitational body, or the
scattering trajectory of a
subatomic particle, among others.
Each
branch of the hyperbola has two arms which become straighter (lower curvature) further out from the center of the hyperbola. Diagonally opposite arms, one from each branch, tend in the limit to a common line, called the
asymptote of those two arms. So there are two asymptotes, whose intersection is at the center of
symmetry of the hyperbola, which can be thought of as the mirror point about which each branch reflects to form the other branch. In the case of the curve $y(x)=1/x$ the asymptotes are the two
coordinate axes.^{
[2]}
The word "hyperbola" derives from the
Greekὑπερβολή, 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]}
Definitions
As locus of points
A hyperbola can be defined geometrically as a
set of points (
locus of points) in the Euclidean plane:
A hyperbola is a set of points, such that for any point $P$ of the set, the absolute difference of the distances $|PF_{1}|,\,|PF_{2}|$ to two fixed points $F_{1},F_{2}$ (the foci) is constant, usually denoted by $2a,\,a>0$:^{
[6]}
The midpoint $M$ of the line segment joining the foci is called the center of the hyperbola.^{
[7]} The line through the foci is called the major axis. It contains the vertices$V_{1},V_{2}$, which have distance $a$ to the center. The distance $c$ of the foci to the center is called the focal distance or linear eccentricity. The quotient ${\tfrac {c}{a}}$ is the eccentricity$e$.
The equation $\left|\left|PF_{2}\right|-\left|PF_{1}\right|\right|=2a$ can be viewed in a different way (see diagram):
If $c_{2}$ is the circle with midpoint $F_{2}$ and radius $2a$, then the distance of a point $P$ of the right branch to the circle $c_{2}$ equals the distance to the focus $F_{1}$:
$|PF_{1}|=|Pc_{2}|.$
$c_{2}$ is called the circular directrix (related to focus $F_{2}$) of the hyperbola.^{
[8]}^{
[9]} In order to get the left branch of the hyperbola, one has to use the circular directrix related to $F_{1}$. This property should not be confused with the definition of a hyperbola with help of a directrix (line) below.
Hyperbola with equation y = A/x
If the xy-coordinate system is
rotated about the origin by the angle $+45^{\circ }$ and new coordinates $\xi ,\eta$ are assigned, then $x={\tfrac {\xi +\eta }{\sqrt {2}}},\;y={\tfrac {-\xi +\eta }{\sqrt {2}}}$.
The rectangular hyperbola ${\tfrac {x^{2}-y^{2}}{a^{2}}}=1$ (whose semi-axes are equal) has the new equation ${\tfrac {2\xi \eta }{a^{2}}}=1$.
Solving for $\eta$ yields $\eta ={\tfrac {a^{2}/2}{\xi }}\ .$
Thus, in an xy-coordinate system the graph of a function $f:x\mapsto {\tfrac {A}{x}},\;A>0\;,$ with equation
$y={\frac {A}{x}}\;,A>0\;,$
is a rectangular hyperbola entirely in the first and third
quadrants with
the coordinate axes as asymptotes,
the line $y=x$ as major axis ,
the center$(0,0)$ and the semi-axis$a=b={\sqrt {2A}}\;,$
the vertices$\left({\sqrt {A}},{\sqrt {A}}\right),\left(-{\sqrt {A}},-{\sqrt {A}}\right)\;,$
the semi-latus rectum and radius of curvature at the vertices $p=a={\sqrt {2A}}\;,$
the linear eccentricity$c=2{\sqrt {A}}$ and the eccentricity $e={\sqrt {2}}\;,$
the tangent$y=-{\tfrac {A}{x_{0}^{2}}}x+2{\tfrac {A}{x_{0}}}$ at point $(x_{0},A/x_{0})\;.$
A rotation of the original hyperbola by $-45^{\circ }$ results in a rectangular hyperbola entirely in the second and fourth quadrants, with the same asymptotes, center, semi-latus rectum, radius of curvature at the vertices, linear eccentricity, and eccentricity as for the case of $+45^{\circ }$ rotation, with equation
$y=-{\frac {A}{x}}\;,~~A>0\;,$
the semi-axes$a=b={\sqrt {2A}}\;,$
the line $y=-x$ as major axis,
the vertices$\left(-{\sqrt {A}},{\sqrt {A}}\right),\left({\sqrt {A}},-{\sqrt {A}}\right)\;.$
Shifting the hyperbola with equation $y={\frac {A}{x}},\ A\neq 0\ ,$ so that the new center is $(c_{0},d_{0})$, yields the new equation
$y={\frac {A}{x-c_{0}}}+d_{0}\;,$
and the new asymptotes are $x=c_{0}$ and $y=d_{0}$. The shape parameters $a,b,p,c,e$ remain unchanged.
By the directrix property
The two lines at distance ${\textstyle d={\frac {a^{2}}{c}}}$ from the center and parallel to the minor axis are called directrices of the hyperbola (see diagram).
For an arbitrary point $P$ of the hyperbola the quotient of the distance to one focus and to the corresponding directrix (see diagram) is equal to the eccentricity:
The proof for the pair $F_{1},l_{1}$ follows from the fact that $|PF_{1}|^{2}=(x-c)^{2}+y^{2},\ |Pl_{1}|^{2}=\left(x-{\tfrac {a^{2}}{c}}\right)^{2}$ and $y^{2}={\tfrac {b^{2}}{a^{2}}}x^{2}-b^{2}$ satisfy the equation
The inverse statement is also true and can be used to define a hyperbola (in a manner similar to the definition of a parabola):
For any point $F$ (focus), any line $l$ (directrix) not through $F$ and any
real number$e$ with $e>1$ the set of points (locus of points), for which the quotient of the distances to the point and to the line is $e$
(The choice $e=1$ yields a
parabola and if $e<1$ an
ellipse.)
Proof
Let $F=(f,0),\ e>0$ and assume $(0,0)$ is a point on the curve.
The directrix $l$ has equation $x=-{\tfrac {f}{e}}$. With $P=(x,y)$, the relation $|PF|^{2}=e^{2}|Pl|^{2}$ produces the equations
$(x-f)^{2}+y^{2}=e^{2}\left(x+{\tfrac {f}{e}}\right)^{2}=(ex+f)^{2}$ and $x^{2}(e^{2}-1)+2xf(1+e)-y^{2}=0.$
The substitution $p=f(1+e)$ yields
$x^{2}(e^{2}-1)+2px-y^{2}=0.$
This is the equation of an ellipse ($e<1$) or a parabola ($e=1$) or a hyperbola ($e>1$). All of these non-degenerate conics have, in common, the origin as a vertex (see diagram).
If $e>1$, introduce new parameters $a,b$ so that $e^{2}-1={\tfrac {b^{2}}{a^{2}}},{\text{ and }}\ p={\tfrac {b^{2}}{a}}$, and then the equation above becomes
which is the equation of a hyperbola with center $(-a,0)$, the x-axis as major axis and the major/minor semi axis $a,b$.
Construction of a directrix
Because of $c\cdot {\tfrac {a^{2}}{c}}=a^{2}$ point $L_{1}$ of directrix $l_{1}$ (see diagram) and focus $F_{1}$ are inverse with respect to the
circle inversion at circle $x^{2}+y^{2}=a^{2}$ (in diagram green). Hence point $E_{1}$ can be constructed using the
theorem of Thales (not shown in the diagram). The directrix $l_{1}$ is the perpendicular to line ${\overline {F_{1}F_{2}}}$ through point $E_{1}$.
Alternative construction of $E_{1}$: Calculation shows, that point $E_{1}$ is the intersection of the asymptote with its perpendicular through $F_{1}$ (see diagram).
As plane section of a cone
The intersection of an upright double cone by a plane not through the vertex with slope greater than the slope of the lines on the cone is a hyperbola (see diagram: red curve). In order to prove the defining property of a hyperbola (see above) one uses two
Dandelin spheres$d_{1},d_{2}$, which are spheres that touch the cone along circles $c_{1}$,$c_{2}$ and the intersecting (hyperbola) plane at points $F_{1}$ and $F_{2}$. It turns out: $F_{1},F_{2}$ are the foci of the hyperbola.
Let $P$ be an arbitrary point of the intersection curve .
The
generatrix of the cone containing $P$ intersects circle $c_{1}$ at point $A$ and circle $c_{2}$ at a point $B$.
The line segments ${\overline {PF_{1}}}$ and ${\overline {PA}}$ are tangential to the sphere $d_{1}$ and, hence, are of equal length.
The line segments ${\overline {PF_{2}}}$ and ${\overline {PB}}$ are tangential to the sphere $d_{2}$ and, hence, are of equal length.
The result is: $|PF_{1}|-|PF_{2}|=|PA|-|PB|=|AB|$ is independent of the hyperbola point $P$, because no matter where point $P$ is, $A,B$ have to be on circles $c_{1}$,$c_{2}$, and line segment $AB$ has to cross the apex. Therefore, as point $P$ moves along the red curve (hyperbola), line segment ${\overline {AB}}$ simply rotates about apex without changing its length.
Pin and string construction
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]}
Choose the foci$F_{1},F_{2}$, the vertices $V_{1},V_{2}$ and one of the circular directrices , for example $c_{2}$ (circle with radius $2a$)
A ruler is fixed at point $F_{2}$ free to rotate around $F_{2}$. Point $B$ is marked at distance $2a$.
A string with length $|AB|$ is prepared.
One end of the string is pinned at point $A$ on the ruler, the other end is pinned to point $F_{1}$.
Take a pen and hold the string tight to the edge of the ruler.
Rotating the ruler around $F_{2}$ prompts the pen to draw an arc of the right branch of the hyperbola, because of $|PF_{1}|=|PB|$ (see the definition of a hyperbola by circular directrices).
Given two
pencils$B(U),B(V)$ of lines at two points $U,V$ (all lines containing $U$ and $V$, respectively) and a projective but not perspective mapping $\pi$ of $B(U)$ onto $B(V)$, then the intersection points of corresponding lines form a non-degenerate projective conic section.
For the generation of points of the hyperbola ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1$ one uses the pencils at the vertices $V_{1},V_{2}$. Let $P=(x_{0},y_{0})$ be a point of the hyperbola and $A=(a,y_{0}),B=(x_{0},0)$. The line segment ${\overline {BP}}$ is divided into n equally-spaced segments and this division is projected parallel with the diagonal $AB$ as direction onto the line segment ${\overline {AP}}$ (see diagram). The parallel projection is part of the projective mapping between the pencils at $V_{1}$ and $V_{2}$ needed. The intersection points of any two related lines $S_{1}A_{i}$ and $S_{2}B_{i}$ are points of the uniquely defined hyperbola.
Remarks:
The subdivision could be extended beyond the points $A$ and $B$ in order to get more points, but the determination of the intersection points would become more inaccurate. A better idea is extending the points already constructed by symmetry (see animation).
The Steiner generation exists for ellipses and parabolas, too.
The Steiner generation is sometimes called a parallelogram method because one can use other points rather than the vertices, which starts with a parallelogram instead of a rectangle.
Inscribed angles for hyperbolas y = a/(x − b) + c and the 3-point-form
A hyperbola with equation $y={\tfrac {a}{x-b}}+c,\ a\neq 0$ is uniquely determined by three points $(x_{1},y_{1}),\;(x_{2},y_{2}),\;(x_{3},y_{3})$ with different x- and y-coordinates. A simple way to determine the shape parameters $a,b,c$ uses the inscribed angle theorem for hyperbolas:
In order to measure an angle between two lines with equations $y=m_{1}x+d_{1},\ y=m_{2}x+d_{2}\ ,m_{1},m_{2}\neq 0$ in this context one uses the quotient
${\frac {m_{1}}{m_{2}}}\ .$
Analogous to the
inscribed angle theorem for circles one gets the
Inscribed angle theorem for hyperbolas^{
[11]}^{
[12]} — For four points $P_{i}=(x_{i},y_{i}),\ i=1,2,3,4,\ x_{i}\neq x_{k},y_{i}\neq y_{k},i\neq k$ (see diagram) the following statement is true:
The four points are on a hyperbola with equation $y={\tfrac {a}{x-b}}+c$ if and only if the angles at $P_{3}$ and $P_{4}$ are equal in the sense of the measurement above. That means if
The proof can be derived by straightforward calculation. If the points are on a hyperbola, one can assume the hyperbola's equation is $y=a/x$.
A consequence of the inscribed angle theorem for hyperbolas is the
3-point-form of a hyperbola's equation — The equation of the hyperbola determined by 3 points $P_{i}=(x_{i},y_{i}),\ i=1,2,3,\ x_{i}\neq x_{k},y_{i}\neq y_{k},i\neq k$ is the solution of the equation
Any hyperbola is the affine image of the unit hyperbola with equation $x^{2}-y^{2}=1$.
Parametric representation
An affine transformation of the Euclidean plane has the form ${\vec {x}}\to {\vec {f}}_{0}+A{\vec {x}}$, where $A$ is a regular
matrix (its
determinant is not 0) and ${\vec {f}}_{0}$ is an arbitrary vector. If ${\vec {f}}_{1},{\vec {f}}_{2}$ are the column vectors of the matrix $A$, the unit hyperbola $(\pm \cosh(t),\sinh(t)),t\in \mathbb {R} ,$ is mapped onto the hyperbola
${\vec {f}}_{0}$ is the center, ${\vec {f}}_{0}+{\vec {f}}_{1}$ a point of the hyperbola and ${\vec {f}}_{2}$ a tangent vector at this point.
Vertices
In general the vectors ${\vec {f}}_{1},{\vec {f}}_{2}$ are not perpendicular. That means, in general ${\vec {f}}_{0}\pm {\vec {f}}_{1}$ are not the vertices of the hyperbola. But ${\vec {f}}_{1}\pm {\vec {f}}_{2}$ point into the directions of the asymptotes. The tangent vector at point ${\vec {p}}(t)$ is
The formulae $\cosh ^{2}x+\sinh ^{2}x=\cosh 2x$,$2\sinh x\cosh x=\sinh 2x$, and $\operatorname {arcoth} x={\tfrac {1}{2}}\ln {\tfrac {x+1}{x-1}}$ were used.
The two vertices of the hyperbola are ${\vec {f}}_{0}\pm \left({\vec {f}}_{1}\cosh t_{0}+{\vec {f}}_{2}\sinh t_{0}\right).$
Implicit representation
Solving the parametric representation for $\cosh t,\sinh t$ by
Cramer's rule and using $\;\cosh ^{2}t-\sinh ^{2}t-1=0\;$, one gets the implicit representation
The definition of a hyperbola in this section gives a parametric representation of an arbitrary hyperbola, even in space, if one allows ${\vec {f}}\!_{0},{\vec {f}}\!_{1},{\vec {f}}\!_{2}$ to be vectors in space.
As an affine image of the hyperbola y = 1/x
Because the unit hyperbola $x^{2}-y^{2}=1$ is affinely equivalent to the hyperbola $y=1/x$, an arbitrary hyperbola can be considered as the affine image (see previous section) of the hyperbola $y=1/x\,$:
$M:{\vec {f}}_{0}$ is the center of the hyperbola, the vectors ${\vec {f}}_{1},{\vec {f}}_{2}$ have the directions of the asymptotes and ${\vec {f}}_{1}+{\vec {f}}_{2}$ is a point of the hyperbola. The tangent vector is
$\left|{\vec {f}}\!_{1}\right|=\left|{\vec {f}}\!_{2}\right|$ is equivalent to $t_{0}=\pm 1$ and ${\vec {f}}_{0}\pm ({\vec {f}}_{1}+{\vec {f}}_{2})$ are the vertices of the hyperbola.
The following properties of a hyperbola are easily proven using the representation of a hyperbola introduced in this section.
Tangent construction
The tangent vector can be rewritten by factorization:
the diagonal $AB$ of the parallelogram $M:\ {\vec {f}}_{0},\ A={\vec {f}}_{0}+{\vec {f}}_{1}t,\ B:\ {\vec {f}}_{0}+{\vec {f}}_{2}{\tfrac {1}{t}},\ P:\ {\vec {f}}_{0}+{\vec {f}}_{1}t+{\vec {f}}_{2}{\tfrac {1}{t}}$ is parallel to the tangent at the hyperbola point $P$ (see diagram).
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]}
Area of the grey parallelogram
The area of the grey parallelogram $MAPB$ in the above diagram is
and hence independent of point $P$. The last equation follows from a calculation for the case, where $P$ is a vertex and the hyperbola in its canonical form ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1\,.$
Point construction
For a hyperbola with parametric representation ${\vec {x}}={\vec {p}}(t)={\vec {f}}_{1}t+{\vec {f}}_{2}{\tfrac {1}{t}}$ (for simplicity the center is the origin) the following is true:
For any two points $P_{1}:\ {\vec {f}}_{1}t_{1}+{\vec {f}}_{2}{\tfrac {1}{t_{1}}},\ P_{2}:\ {\vec {f}}_{1}t_{2}+{\vec {f}}_{2}{\tfrac {1}{t_{2}}}$ the points
are collinear with the center of the hyperbola (see diagram).
The simple proof is a consequence of the equation ${\tfrac {1}{t_{1}}}{\vec {a}}={\tfrac {1}{t_{2}}}{\vec {b}}$.
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]}
Tangent–asymptotes triangle
For simplicity the center of the hyperbola may be the origin and the vectors ${\vec {f}}_{1},{\vec {f}}_{2}$ have equal length. If the last assumption is not fulfilled one can first apply a parameter transformation (see above) in order to make the assumption true. Hence $\pm ({\vec {f}}_{1}+{\vec {f}}_{2})$ are the vertices, $\pm ({\vec {f}}_{1}-{\vec {f}}_{2})$ span the minor axis and one gets $|{\vec {f}}_{1}+{\vec {f}}_{2}|=a$ and $|{\vec {f}}_{1}-{\vec {f}}_{2}|=b$.
For the intersection points of the tangent at point ${\vec {p}}(t_{0})={\vec {f}}_{1}t_{0}+{\vec {f}}_{2}{\tfrac {1}{t_{0}}}$ with the asymptotes one gets the points
(see rules for
determinants).
$\left|\det({\vec {f}}_{1},{\vec {f}}_{2})\right|$ is the area of the rhombus generated by ${\vec {f}}_{1},{\vec {f}}_{2}$. The area of a rhombus is equal to one half of the product of its diagonals. The diagonals are the semi-axes $a,b$ of the hyperbola. Hence:
The area of the triangle $MCD$ is independent of the point of the hyperbola: $A=ab.$
Reciprocation of a circle
The
reciprocation of a
circleB 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
$e={\frac {\overline {BC}}{r}}.$
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.
Quadratic equation
A hyperbola can also be defined as a second-degree equation in the Cartesian coordinates $(x,y)$ in the
plane,
This determinant $\Delta$ is sometimes called the discriminant of the conic section.^{
[16]}
The general equation's coefficients can be obtained from known semi-major axis $a,$ semi-minor axis $b,$ center coordinates $(x_{\circ },y_{\circ })$, and rotation angle $\theta$ (the angle from the positive horizontal axis to the hyperbola's major axis) using the formulae:
the left focus is $(-ae,0)$ and the right focus is $(ae,0),$ where $e$ is the eccentricity. Denote the distances from a point $(x,y)$ to the left and right foci as $r_{1}$ and $r_{2}.$ For a point on the right branch,
$r_{1}-r_{2}=2a,$
and for a point on the left branch,
$r_{2}-r_{1}=2a.$
This can be proved as follows:
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
the foci are the points $F_{1}=(c,0),\ F_{2}=(-c,0)$,^{
[17]}
the vertices are $V_{1}=(a,0),\ V_{2}=(-a,0)$.^{
[18]}
For an arbitrary point $(x,y)$ the distance to the focus $(c,0)$ is ${\textstyle {\sqrt {(x-c)^{2}+y^{2}}}}$ and to the second focus ${\textstyle {\sqrt {(x+c)^{2}+y^{2}}}}$. Hence the point $(x,y)$ is on the hyperbola if the following condition is fulfilled
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).
The axes of
symmetry or principal axes are the transverse axis (containing the segment of length 2a with endpoints at the vertices) and the conjugate axis (containing the segment of length 2b perpendicular to the transverse axis and with midpoint at the hyperbola's center).^{
[19]} As opposed to an ellipse, a hyperbola has only two vertices: $(a,0),\;(-a,0)$. The two points $(0,b),\;(0,-b)$ on the conjugate axes are not on the hyperbola.
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.
Eccentricity
For a hyperbola in the above canonical form, the
eccentricity is given by
Solving the equation (above) of the hyperbola for $y$ yields
$y=\pm {\frac {b}{a}}{\sqrt {x^{2}-a^{2}}}.$
It follows from this that the hyperbola approaches the two lines
$y=\pm {\frac {b}{a}}x$
for large values of $|x|$. These two lines intersect at the center (origin) and are called asymptotes of the hyperbola ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1\ .$^{
[20]}
With the help of the second figure one can see that
${\color {blue}{(1)}}$ The perpendicular distance from a focus to either asymptote is $b$ (the semi-minor axis).
From the
Hesse normal form${\tfrac {bx\pm ay}{\sqrt {a^{2}+b^{2}}}}=0$ of the asymptotes and the equation of the hyperbola one gets:^{
[21]}
${\color {magenta}{(2)}}$ The product of the distances from a point on the hyperbola to both the asymptotes is the constant ${\tfrac {a^{2}b^{2}}{a^{2}+b^{2}}}\ ,$ which can also be written in terms of the eccentricity e as $\left({\tfrac {b}{e}}\right)^{2}.$
From the equation $y=\pm {\frac {b}{a}}{\sqrt {x^{2}-a^{2}}}$ of the hyperbola (above) one can derive:
${\color {green}{(3)}}$ The product of the slopes of lines from a point P to the two vertices is the constant $b^{2}/a^{2}\ .$
In addition, from (2) above it can be shown that^{
[21]}
${\color {red}{(4)}}$The product of the distances from a point on the hyperbola to the asymptotes along lines parallel to the asymptotes is the constant ${\tfrac {a^{2}+b^{2}}{4}}.$
Semi-latus rectum
The length of the chord through one of the foci, perpendicular to the major axis of the hyperbola, is called the latus rectum. One half of it is the semi-latus rectum$p$. A calculation shows
$p={\frac {b^{2}}{a}}.$
The semi-latus rectum $p$ may also be viewed as the radius of curvature at the vertices.
Tangent
The simplest way to determine the equation of the tangent at a point $(x_{0},y_{0})$ is to
implicitly differentiate the equation ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1$ of the hyperbola. Denoting dy/dx as y′, this produces
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 2f. The tangent to the hyperbola at P intersects that axis at point Q at an angle ∠PQV of greater than 45°.
Rectangular hyperbola
In the case $a=b$ the hyperbola is called rectangular (or equilateral), because its asymptotes intersect at right angles. For this case, the linear eccentricity is $c={\sqrt {2}}a$, the eccentricity $e={\sqrt {2}}$ and the semi-latus rectum $p=a$. The graph of the equation $y=1/x$ is a rectangular hyperbola.
Parametric representation with hyperbolic sine/cosine
Using the
hyperbolic sine and cosine functions$\cosh ,\sinh$, a parametric representation of the hyperbola ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1$ can be obtained, which is similar to the parametric representation of an ellipse:
$(\pm a\cosh t,b\sinh t),\,t\in \mathbb {R} \ ,$
which satisfies the Cartesian equation because $\cosh ^{2}t-\sinh ^{2}t=1.$
Further parametric representations are given in the section
Parametric equations below.
Conjugate hyperbola
Exchange ${\frac {x^{2}}{a^{2}}}$ and ${\frac {y^{2}}{b^{2}}}$ to obtain the equation of the conjugate hyperbola (see diagram):
A hyperbola and its conjugate may have
diameters which are conjugate. In the theory of
special relativity, such diameters may represent axes of time and space, where one hyperbola represents
events at a given spatial distance from the
center, and the other represents events at a corresponding temporal distance from the center.
In polar coordinates
Origin at the focus
The polar coordinates used most commonly for the hyperbola are defined relative to the Cartesian coordinate system that has its origin in a focus and its x-axis pointing towards the origin of the "canonical coordinate system" as illustrated in the first diagram.
In this case the angle $\varphi$ is called true anomaly.
With the tangent slope as parameter: A parametric representation, which uses the slope $m$ of the tangent at a point of the hyperbola can be obtained analogously to the ellipse case: Replace in the ellipse case $b^{2}$ by $-b^{2}$ and use formulae for the
hyperbolic functions. One gets
Here, ${\vec {c}}_{-}$ is the upper, and ${\vec {c}}_{+}$ the lower half of the hyperbola. The points with vertical tangents (vertices $(\pm a,0)$) are not covered by the representation. The equation of the tangent at point ${\vec {c}}_{\pm }(m)$ is
$y=mx\pm {\sqrt {m^{2}a^{2}-b^{2}}}.$
This description of the tangents of a hyperbola is an essential tool for the determination of the
orthoptic of a hyperbola.
Let $a$ be twice the area between the $x$ axis and a ray through the origin intersecting the unit hyperbola, and define ${\textstyle (x,y)=(\cosh a,\sinh a)=(x,{\sqrt {x^{2}-1}})}$ as the coordinates of the intersection point.
Then the area of the hyperbolic sector is the area of the triangle minus the curved region past the vertex at $(1,0)$:
The tangent at a point $P$ bisects the angle between the lines ${\overline {PF_{1}}},{\overline {PF_{2}}}.$ This is called the optical property or reflection property of a hyperbola.^{
[23]}
Proof
Let $L$ be the point on the line ${\overline {PF_{2}}}$ with the distance $2a$ to the focus $F_{2}$ (see diagram, $a$ is the semi major axis of the hyperbola). Line $w$ is the bisector of the angle between the lines ${\overline {PF_{1}}},{\overline {PF_{2}}}$. In order to prove that $w$ is the tangent line at point $P$, one checks that any point $Q$ on line $w$ which is different from $P$ cannot be on the hyperbola. Hence $w$ has only point $P$ in common with the hyperbola and is, therefore, the tangent at point $P$.
From the diagram and the
triangle inequality one recognizes that $|QF_{2}|<|LF_{2}|+|QL|=2a+|QF_{1}|$ holds, which means: $|QF_{2}|-|QF_{1}|<2a$. But if $Q$ is a point of the hyperbola, the difference should be $2a$.
Midpoints of parallel chords
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.
The proof of the property on midpoints is best done for the hyperbola $y=1/x$. Because any hyperbola is an affine image of the hyperbola $y=1/x$ (see section below) and an affine transformation preserves parallelism and midpoints of line segments, the property is true for all hyperbolas:
For two points $P=\left(x_{1},{\tfrac {1}{x_{1}}}\right),\ Q=\left(x_{2},{\tfrac {1}{x_{2}}}\right)$ of the hyperbola $y=1/x$
the midpoint of the chord is $M=\left({\tfrac {x_{1}+x_{2}}{2}},\cdots \right)=\cdots ={\tfrac {x_{1}+x_{2}}{2}}\;\left(1,{\tfrac {1}{x_{1}x_{2}}}\right)\ ;$
the slope of the chord is ${\frac {{\tfrac {1}{x_{2}}}-{\tfrac {1}{x_{1}}}}{x_{2}-x_{1}}}=\cdots =-{\tfrac {1}{x_{1}x_{2}}}\ .$
For parallel chords the slope is constant and the midpoints of the parallel chords lie on the line $y={\tfrac {1}{x_{1}x_{2}}}\;x\ .$
Consequence: for any pair of points $P,Q$ of a chord there exists a skew reflection with an axis (set of fixed points) passing through the center of the hyperbola, which exchanges the points $P,Q$ and leaves the hyperbola (as a whole) fixed. A skew reflection is a generalization of an ordinary reflection across a line $m$, where all point-image pairs are on a line perpendicular to $m$.
Because a skew reflection leaves the hyperbola fixed, the pair of asymptotes is fixed, too. Hence the midpoint $M$ of a chord $PQ$ divides the related line segment ${\overline {P}}\,{\overline {Q}}$ between the asymptotes into halves, too. This means that $|P{\overline {P}}|=|Q{\overline {Q}}|$. This property can be used for the construction of further points $Q$ of the hyperbola if a point $P$ and the asymptotes are given.
If the chord degenerates into a tangent, then the touching point divides the line segment between the asymptotes in two halves.
For a hyperbola ${\textstyle {\frac {x^{2}}{a^{2}}}-{\frac {y^{2}}{b^{2}}}=1,\,a>b}$ the intersection points of orthogonal tangents lie on the circle $x^{2}+y^{2}=a^{2}-b^{2}$.
This circle is called the orthoptic of the given hyperbola.
The tangents may belong to points on different branches of the hyperbola.
In case of $a\leq b$ there are no pairs of orthogonal tangents.
Pole-polar relation for a hyperbola
Any hyperbola can be described in a suitable coordinate system by an equation ${\tfrac {x^{2}}{a^{2}}}-{\tfrac {y^{2}}{b^{2}}}=1$. The equation of the tangent at a point $P_{0}=(x_{0},y_{0})$ of the hyperbola is ${\tfrac {x_{0}x}{a^{2}}}-{\tfrac {y_{0}y}{b^{2}}}=1.$ If one allows point $P_{0}=(x_{0},y_{0})$ to be an arbitrary point different from the origin, then
point $P_{0}=(x_{0},y_{0})\neq (0,0)$ is mapped onto the line ${\frac {x_{0}x}{a^{2}}}-{\frac {y_{0}y}{b^{2}}}=1$, not through the center of the hyperbola.
This relation between points and lines is a
bijection.
line $y=mx+d,\ d\neq 0$ onto the point $\left(-{\frac {ma^{2}}{d}},-{\frac {b^{2}}{d}}\right)$ and
line $x=c,\ c\neq 0$ onto the point $\left({\frac {a^{2}}{c}},0\right)\ .$
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:
For a point (pole) on the hyperbola the polar is the tangent at this point (see diagram: $P_{1},\ p_{1}$).
For a pole $P$outside the hyperbola the intersection points of its polar with the hyperbola are the tangency points of the two tangents passing $P$ (see diagram: $P_{2},\ p_{2},\ P_{3},\ p_{3}$).
For a point within the hyperbola the polar has no point with the hyperbola in common. (see diagram: $P_{4},\ p_{4}$).
Remarks:
The intersection point of two polars (for example: $p_{2},p_{3}$) is the pole of the line through their poles (here: $P_{2},P_{3}$).
The foci $(c,0),$ and $(-c,0)$ respectively and the directrices $x={\tfrac {a^{2}}{c}}$ and $x=-{\tfrac {a^{2}}{c}}$ respectively belong to pairs of pole and polar.
Pole-polar relations exist for ellipses and parabolas, too.
Other properties
The following are
concurrent: (1) a circle passing through the hyperbola's foci and centered at the hyperbola's center; (2) either of the lines that are tangent to the hyperbola at the vertices; and (3) either of the asymptotes of the hyperbola.^{
[24]}^{
[25]}
The following are also concurrent: (1) the circle that is centered at the hyperbola's center and that passes through the hyperbola's vertices; (2) either directrix; and (3) either of the asymptotes.^{
[25]}
Arc length
The arc length of a hyperbola does not have an
elementary expression. The upper half of a hyperbola can be parameterized as
$y=b{\sqrt {{\frac {x^{2}}{a^{2}}}-1}}.$
Then the integral giving the arc length $s$ from $x_{1}$ to $x_{2}$ can be computed 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.
Elliptic coordinates
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.
Conic section analysis of the hyperbolic appearance of circles
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.
The image of a circle c is
a circle, if circle c is in a special position, for example parallel to the image plane and others (see stereographic projection),
an ellipse, if c has no point with the lens plane in common,
a parabola, if c has one point with the lens plane in common and
a hyperbola, if c has two points with the lens plane in common.
(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.
Applications
Sundials
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.
Multilateration
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 2a from two given points is a hyperbola of vertex separation 2a whose foci are the two given points.
Path followed by a particle
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.
Korteweg–de Vries equation
The hyperbolic trig function $\operatorname {sech} \,x$ appears as one solution to the
Korteweg–de Vries equation which describes the motion of a soliton wave in a canal.
Angle trisection
As shown first by
Apollonius of Perga, a hyperbola can be used to
trisect any angle, a well studied problem of geometry. Given an angle, first draw a circle centered at its vertex O, which intersects the sides of the angle at points A and B. Next draw the line segment with endpoints A and B and its perpendicular bisector $\ell$. Construct a hyperbola of
eccentricitye=2 with $\ell$ as
directrix and B as a focus. Let P be the intersection (upper) of the hyperbola with the circle. Angle POB trisects angle AOB.
To prove this, reflect the line segment OP about the line $\ell$ obtaining the point P' as the image of P. Segment AP' has the same length as segment BP due to the reflection, while segment PP' has the same length as segment BP due to the eccentricity of the hyperbola. As OA, OP', OP and OB are all radii of the same circle (and so, have the same length), the triangles OAP', OPP' and OPB are all congruent. Therefore, the angle has been trisected, since 3×POB = AOB.^{
[27]}
Efficient portfolio frontier
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.
^Boyer, Carl B.;
Merzbach, Uta C. (2011),
A History of Mathematics, Wiley, p. 73,
ISBN9780470630563, It was Apollonius (possibly following up a suggestion of Archimedes) who introduced the names "ellipse" and "hyperbola" in connection with these curves.
^Eves, Howard (1963), A Survey of Geometry (Vol. One), Allyn and Bacon, pp. 30–31
^Apostol, Tom M.; Mnatsakanian, Mamikon A. (2012), New Horizons in Geometry, The Dolciani Mathematical Expositions #47, The Mathematical Association of America, p. 251,
ISBN978-0-88385-354-2
^The German term for this circle is Leitkreis which can be translated as "Director circle", but that term has a different meaning in the English literature (see
Director circle).
^Korn, Granino A;
Korn, Theresa M. (2000). Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review (second ed.). Dover Publ. p. 40.
^
^{a}^{b}Mitchell, Douglas W., "A property of hyperbolas and their asymptotes", Mathematical Gazette 96, July 2012, 299–301.
^J. W. Downs, Practical Conic Sections, Dover Publ., 2003 (orig. 1993): p. 26.
^ Coffman, R. T.; Ogilvy, C. S. (1963), "The 'Reflection Property' of the Conics", Mathematics Magazine, 36 (1): 11–12,
doi:
10.2307/2688124 Flanders, Harley (1968), "The Optical Property of the Conics", American Mathematical Monthly, 75 (4): 399,
doi:
10.2307/2313439
Brozinsky, Michael K. (1984), "Reflection Property of the Ellipse and the Hyperbola", College Mathematics Journal, 15 (2): 140–42,
doi:
10.2307/2686519
^"Hyperbola". Mathafou.free.fr. Archived from
the original on 4 March 2016. Retrieved 26 August 2018.