HYPERBOLA

This article appears in Volume V14, Page 199 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: HOR-I25

HYPERBOLA , a conic section, consisting of two open branches, each extending to infinity. It may be defined in several ways. The in solido definition as the section of a

cone

CONE (Gr. Kwvos)

by a plane at a less inclination to the

axis

AXIS (Lat. for " axle ")

than the generator brings out the existence of the two infinite branches if we imagine the

cone

CONE (Gr. Kwvos)

to be

double

and to extend to infinity. The in plane definition, i.e. as the conic having an eccentricity greater than unity, is a convenient starting-point for the Euclidian investigation. In projective geometry it may be defined as the conic which inter-sects the line at infinity in two real points, or to which it is possible to draw two real tangents from the centre. Analytically, it is defined by an equation of the second degree, of which the highest terms have real roots (see CONIC SECTION).
While resembling the

parabola

PARABOLA

in extending to infinity, the curve has closest affinities to the ellipse. Thus it has a real centre, two foci, two directrices and two vertices; the transverse

axis

AXIS (Lat. for " axle ")

, joining the vertices, corresponds to the major axis of the ellipse, and the line through the centre and perpendicular to this axis is called the conjugate axis, and corresponds to the

minor

axis of the ellipse; about these axes the curve is symmetrical. The curve does not appear to intersect the conjugate axis, but the introduction of imagmaries permits us to regard it as cutting this axis in two unreal points. Calling the foci S, S', the real vertices A, A', the extremities
199
of the conjugate axis B, B' and the centre C, the positions of B, B' are given by AB=AB'=CS. If a rectangle be constructed about AA' and BB', the diagonals of this figure are the " asymptotes " of the curve; they are the tangents from the centre, and hence

touch

TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw)

the curve at infinity. These two lines may be pictured in the in solido definition as the section of a cone by a plane through its vertex and parallel to the plane generating the hyperbola. If the asymptotes be perpendicular, or, in other words, the

principal

PRINCIPAL

axes be equal, the curve is called the rectangular hyperbola; The hyper-bola which has for its transverse and conjugate axes the transverse and conjugate axes of another hyperbola is said to be the conjugate hyperbola.
Some properties of the curve will be briefly stated: If PN be the ordinate of the point P on the curve, AA' the vertices, X the meet of the directrix and axis and C the centre, then PN2: AN.NA': : SX2: AX.A'X, i.e. PN2 is to AN.NA' in a constant ratio. The circle on AA' as

diameter

DIAMETER (from the Cr. &a, through, µErpov, measure)

is called the auxiliarly circle; obviously AN.NA' equals the square of the tangent to this circle from N, and hence the ratio of PN to the tangent to the auxiliarly circle from N equals the ratio of the conjugate axis to the transverse. We may observe that the asymptotes intersect this circle in the same points as the directrices. An important property is: the difference of the focal distances of any point on the curve equals the transverse axis. The tangent at any point bisects the

angle

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

between the focal distances of the point, and the normal is equally inclined to the focal distances. Also the auxiliarly circle is the locus of the feet of the perpendiculars from the foci on any tangent. Two tangents from any point are equally inclined to the focal distance of the point. If the tangent at P meet the conjugate axis in t, and the transverse in N, then Ct. PN = BC2; similarly if g and G be the corresponding inter-sections of the normal, PG : Pg : : BC2 : AC2. A

diameter

DIAMETER (from the Cr. &a, through, µErpov, measure)

is a line through the centre and terminated by the curve : it bisects all chords parallel to the tangents at its extremities; the diameter parallel to these chords is its conjugate diameter. Any diameter is a mean proportional between the transverse axis and the focal chord parallel to the diameter. Any line cuts off equal distances between the curve and the asymptotes. If the tangent at P meets the asymptotes in R, R', then CR.CR' = CS2. The geometry of the rectangular hyper-bola is simplified by the fact that its

principal

PRINCIPAL

axes are equal.
Analytically the hyperbola is given by axe+zhxy+bye+2gx+ 2fy+c=o wherein ab>h2. Referred to the centre this becomes Axe+2Hxy+By2+C=o; and if the axes of coordinates be the principal axes of the curve, the equation is further simplified to Ax2-By2=C, or if the semi-transverse axis be a, and the semi-conjugate b, x2/a2-y2/b2=1. This is the most commonly used form. In the rectangular hyperbola a=b; hence its equation isx2-y2=o. The equations to the asymptotes are x/a = * y/b and x= *y respectively. Referred to the asymptotes as axes the general equation becomes xy=k2; obviously the axes are oblique in the general hyperbola and rectangular in the rectangular hyperbola. The values of the constant k2 are 1(a2+b2) and 2a2 respectively. (See GEOMETRY: Analytical; Projective.)

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