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Introduction to Hyperbola

MathsHyperbolaFor JEE aspirants

A hyperbola is the locus of a point that moves in a plane so that the ratio of its distance from a fixed point (the focus) to its distance from a fixed line (the directrix) is a constant . Its standard equation is , where . This page explains every basic term of the hyperbola, including eccentricity, foci, directrices, axes, vertices and latus rectum, along with the conjugate hyperbola, auxiliary circle, parametric form and position of a point, with solved examples for JEE Main and JEE Advanced.

Key Formulas - Quick Reference
  1. Standard equation: , with
  2. Eccentricity: , always
  3. Foci ; directrices ; vertices
  4. Transverse axis ; conjugate axis
  5. Latus rectum ; its ends are
  6. Focal distances: , , so
  7. Conjugate hyperbola ; if are the two eccentricities,
  8. Auxiliary circle ; parametric point
  9. Position of : is (inside), (on), (outside)

1. Definition of a Hyperbola

Hyperbolic curves are of special importance in science and technology, especially in astronomy and space studies. For example, a body that passes the Sun only once, without being captured into orbit, moves along a hyperbolic path. In this chapter we study the characteristics of such curves.

Definition: A hyperbola is the locus of a point moving in a plane such that the ratio of its distance from a fixed point to its distance from a fixed line is a constant greater than 1. The fixed point (focus) does not lie on the fixed line (directrix).

Focus-directrix definition of a hyperbola A point P on a hyperbola, its distance PS from the focus S and its perpendicular distance PM from the directrix. The ratio PS to PM equals the eccentricity e, which is greater than 1. x Directrix M P S (focus) A PS = e · PM e > 1
Figure 1: Focus-directrix definition. For every point on the hyperbola, .

Hyperbola as a conic section

A hyperbola is one of the conic sections, the curves obtained when a plane cuts a right circular double cone. Let be the semi-vertical angle of the cone and the angle between the cutting plane and the axis of the cone. When , the plane cuts both nappes of the cone and the section is a hyperbola.

Hyperbola as a section of a double cone Side view of a right circular double cone with vertex V and vertical axis. A cutting plane makes an angle beta with the axis that is smaller than the semi-vertical angle alpha, so it cuts both the upper and lower nappes and the section is a hyperbola. axis V upper nappe lower nappe cutting plane α β Hyperbola when β < α (plane cuts both nappes)
Figure 2: A plane making angle with the axis of a double cone, with , cuts both nappes. The section is a hyperbola.
Angle between plane and axisSection obtained
Circle
Ellipse
Parabola
Hyperbola

2. Standard Equation of a Hyperbola

Take the focus as and the directrix as , where . For any point on the curve, , and the definition gives

Standard equation of a hyperbola:

Because the equation contains only even powers of and , the curve is symmetric about both axes. So and act as a second focus and a second directrix.

Standard hyperbola with foci, vertices, directrices and latus rectum The standard hyperbola x squared over a squared minus y squared over b squared equals 1 with centre C at the origin, vertices A and A prime at plus and minus a, foci S and S prime at plus and minus a e, directrices x equals plus and minus a over e, conjugate axis end points B and B prime, and latus rectum end points L, L prime, M and M prime. x = a/e x = −a/e L L′ M M′ A A′ S(ae, 0) S′(−ae, 0) B(0, b) B′(0, −b) C A(a, 0) A′(−a, 0)
Figure 3: Standard hyperbola with centre , vertices , foci , directrices , conjugate axis and latus rectum .

Basic terms of the hyperbola

TermMeaningFor
Eccentricity The constant ratio , always greater than 1
FociThe two fixed points and
DirectricesThe fixed lines, one for each focus and
Transverse axisThe line segment of length on which both foci and lieAlong the -axis, length
Conjugate axisThe line segment of length between and Along the -axis, length
Principal axesThe transverse and conjugate axes taken togetherThe -axis and -axis
VerticesPoints where the hyperbola meets its transverse axis and
CentreThe point that bisects every chord of the conic drawn through it
Focal chordA chord that passes through a focusAny chord through or
Double ordinateA chord perpendicular to the transverse axisA chord parallel to the -axis
Latus rectumA focal chord perpendicular to the transverse axisLies on , length

Length of the latus rectum

Putting in the equation gives , so . Hence the length of the latus rectum is

Note:

  • (i) Length of latus rectum (distance of the focus from the corresponding directrix). Here the distance is .
  • (ii) The end points of the latera recta are , , and .

General note: The fundamental equation of a hyperbola differs from that of an ellipse only in having in place of . So many propositions for the hyperbola can be obtained from those for the ellipse simply by changing the sign of .

Focal distances of a point

For a point on the right branch, for the directrix . Using for each focus-directrix pair:

So . On the left branch the roles swap and . In both cases

This gives an equivalent definition: a hyperbola is the locus of a point whose distances from two fixed points (the foci) differ by a constant.

Solved Example 1
Find the equation of the hyperbola whose directrix is , focus is and eccentricity is .
Solution:

Let be any point on the hyperbola and draw perpendicular from to the directrix. By definition, , so .

This is the required hyperbola: .

Solved Example 2
Find the eccentricity of the hyperbola whose latus rectum is half of its transverse axis.
Solution:

Let the hyperbola be . Then transverse axis and latus rectum . According to the question,

Hence the required eccentricity is .

3. Hyperbola with Centre at

When the centre is shifted to and the axes stay parallel to the coordinate axes, replace by and by . All the results of the standard form carry over after this shift.

Property
Centre
Transverse axisAlong , length Along , length
Vertices
Foci
Directrices
Eccentricity
Latus rectum

4. Conjugate Hyperbola

Two hyperbolas such that the transverse and conjugate axes of one are respectively the conjugate and transverse axes of the other are called conjugate hyperbolas of each other.

Conjugate hyperbola with transverse axis along the y-axis The conjugate hyperbola y squared over b squared minus x squared over a squared equals 1. Its branches open upward and downward, its vertices are at 0 plus or minus b, its foci S and S prime are at 0 plus or minus b e and its directrices are the horizontal lines y equals plus or minus b over e. x y y = b/e (directrix) y = −b/e (directrix) S(0, be) S′(0, −be) B B′ C Transverse axis: y-axis Conjugate axis: x-axis
Figure 4: Conjugate hyperbola . The transverse axis lies along the -axis, foci are and directrices are .

For the conjugate hyperbola the transverse axis lies along the -axis, so the roles of and are exchanged:

PropertyHyperbola Conjugate
Transverse axis, along -axis, along -axis
Conjugate axis, along -axis, along -axis
Relation
Eccentricity
Vertices
Foci
Directrices
Latus rectum

Note:

  • (a) If and are the eccentricities of a hyperbola and its conjugate, then . This follows because and .
  • (b) The foci of a hyperbola and its conjugate are concyclic and form the vertices of a square. All four foci are at distance from the centre.
  • (c) Two hyperbolas are said to be similar if they have the same eccentricity.
  • (d) Two similar hyperbolas are said to be equal if they have the same latus rectum.
  • (e) If a hyperbola is equilateral, then its conjugate hyperbola is also equilateral.
Solved Example 3
Find the lengths of the transverse axis and conjugate axis, the eccentricity, the coordinates of the foci and vertices, the length of the latus rectum and the equations of the directrices of the hyperbola .
Solution:

Dividing by 144, the equation becomes , which is of the form (a conjugate-type hyperbola). So , giving .

  • Length of transverse axis:
  • Length of conjugate axis:
  • Eccentricity:
  • Foci: , that is,
  • Vertices: , that is,
  • Length of latus rectum:
  • Directrices: , that is,

5. Auxiliary Circle

The circle drawn with centre and the transverse axis as diameter is called the auxiliary circle of the hyperbola. For its equation is

In the figure below, is the foot of the ordinate of a point on the hyperbola and is a tangent to the auxiliary circle. The points and are called the corresponding points of the hyperbola and the auxiliary circle. Since and , we get where .

Auxiliary circle and corresponding points of a hyperbola The auxiliary circle x squared plus y squared equals a squared drawn on the transverse axis as diameter. From N, the foot of the ordinate of P, a tangent NQ is drawn to the circle. P with coordinates a sec theta, b tan theta on the hyperbola and Q with coordinates a cos theta, a sin theta on the circle are corresponding points, and angle QCN equals theta. x y θ P(a sec θ, b tan θ) Q(a cos θ, a sin θ) Q N C A A′
Figure 5: Auxiliary circle . The points on the hyperbola and on the circle are corresponding points.

6. Parametric Representation

The equations and together represent the hyperbola , where is a parameter.

This works because . Values give the right branch and give the left branch.

Note: If is on the hyperbola, then its corresponding point is on the auxiliary circle. The angle is called the eccentric angle of .

7. Position of a Point with Respect to a Hyperbola

For the point , the quantity

is positive, zero or negative according as the point lies inside, on or outside the hyperbola. Here "inside" means the region between a branch and its focus.

Solved Example 4
Find the position of the point relative to the hyperbola .
Solution:

Here .

So the point lies inside the hyperbola .

8. Practice Problems

  1. Find the equation of the hyperbola whose foci are and and eccentricity is 2.
    Answer:
  2. Obtain the equation of a hyperbola with the coordinate axes as principal axes, given that the distances of one of its vertices from the foci are 9 and 1 units.
    Answer: or
  3. The foci of a hyperbola coincide with the foci of the ellipse . Find the equation of the hyperbola if its eccentricity is 2.
    Answer:

Common Mistakes to Avoid

Watch out
  • Using the ellipse relation . For a hyperbola it is , so is always greater than 1.
  • Assuming . In a hyperbola can be smaller than, equal to or larger than . The transverse axis is decided by which squared term is positive, not by which denominator is larger.
  • For , taking the transverse axis as . The right-hand side is , so the transverse axis is along the -axis with length .
  • Writing for a hyperbola and its conjugate. The correct relation is .
  • Reading as "outside". For a hyperbola, means the point lies inside, in the region containing a focus. This is the opposite sign convention to the ellipse.
  • Using as the parametric point. That belongs to the ellipse; the hyperbola uses .
  • Forgetting to add the centre in a shifted hyperbola: the foci of are , not .

Frequently Asked Questions

What is a hyperbola in coordinate geometry?

A hyperbola is the locus of a point whose distance from a fixed point (focus) bears a constant ratio to its distance from a fixed line (directrix). Equivalently, the difference of its distances from the two foci is constant and equal to . Its standard equation is .

How do you find the eccentricity of a hyperbola?

For , use , where is the semi-transverse axis. For the conjugate form , use . The value always exceeds 1, and it equals when .

What is the difference between the transverse axis and the conjugate axis?

The transverse axis is the segment joining the two vertices; both foci lie on its line and its length is for . The conjugate axis is the perpendicular segment through the centre joining , with length . The curve never meets its conjugate axis.

What is the length of the latus rectum of a hyperbola?

The latus rectum is the focal chord perpendicular to the transverse axis. For its length is , which also equals and the square of the conjugate axis divided by the transverse axis. Its end points are .

What is a conjugate hyperbola?

The conjugate of is , whose transverse and conjugate axes are swapped. Both share the same centre and asymptotes, their four foci form a square, and their eccentricities satisfy .

Why does a hyperbola use sec and tan in its parametric form?

Substituting and into gives , so every value of gives a point on the curve. Geometrically, is the angle at the centre to the corresponding point on the auxiliary circle.

Is the hyperbola in the JEE Main syllabus?

Yes. The JEE Main syllabus includes equations of the parabola, ellipse and hyperbola in standard form, and questions on eccentricity, foci, latus rectum and the conjugate hyperbola are common. Every result on this page is directly usable in JEE Main problems.

Which basic hyperbola results matter most for JEE Advanced?

JEE Advanced lists foci, directrices, eccentricity and parametric equations of the hyperbola, followed by tangents, normals and locus problems. Be fluent with , the focal distance property , the parametric point and the sign of , since multi-step problems build on them.

Previous year questions on Introduction to Hyperbola

24 questions from past papers, each with a step-by-step solution.

Show all 24 questions

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