HYPERBOLA-1 (Equation of Hyperbola)

Definition 1 : The locus of (a) point in (a) plane, the difference of whose distance from two fixed points in the plane is constant.

According to definition

|PF2PF1|= constant

Two fixed points are known as Foci of the hyperbola.

The mid point of the line segment joining the foci is called the centre. The line joining the vertices is known as transverse axis and the line through the centre and perpendicular to transverse axis is known as conjugate axis. The point at which the hyperbola intersect the transverse axis is known as vertices of the hyperbola.

Let distance between two foci be 2(c) and distance between two vertices be 2(a) and (a) quantity ’ (b) ’ be (b)2=(c)2(a)2.

Let (a) point P on the hyperbola be (x,y), then according to definition of hyperbola (x+(c))2+y2 (x(c))2+y2=2(a) (x+(c))2+y2=2(a)+(x(c))2+y2

(x+(c))2+y2=4(a)2+(x(c))2+y2+4(a)(x(c))2+y2[2(a)<2(c)(a)<(c)]

4(c)x4(a)2=4(a)(x(c))2+y2

(c)x(a)(a)=(x(c))2+y2

(c)2x2(a)2+(a)22cx=x2+(c)22cx+y2

((c)2(a)2)(a)2x2y2=((c)2(a)2)

x2(a)2y2(c)2(a)2=1x2(a)2y2(b)2=1

x2(a)2y2(b)2=1

y=±(b)(a)x2(a)2

y is real when x2(a)2

y is real when x(a) or x(a)

Definition 2: The locus of (a) point which moves in (a) plane such that the ratio of its distance from (a) fixed point to its perpendicular distance from (a) fixed straight line (not passing through given fixed point) is always constant and greater than 1 .

PSPM=e>1

The equation of hyperbola whose focus is the point (h,k) and directrix is (a)x+(b)y+(c)=0 and whose eccentricity is e, is

(xh)2+(yk)2=e2(ax+by+(c))2(a)2+(b)2

Example: 1 Equation of hyperbola whose focus is (1,2) equations of directrix is x+y+1=0 and eccentricity is 32 is

(a) x2+y218xyy31=0

(b) x2+y2+18xy+34x+50y31=0

(c) x2+y218xyx+y31=0

(d) x2+y2+18xy+x+y+31=0

Show Answer

Solution:

Let variable point P(x,y) then PS=ePM

(x1)2+(y2)2=32{x+y+12}

x2+y22x4y+5=98(x2+y2+2xy+2x+2y+1)

x2+y2+18xy+34x+50y31=0

Answer: (b)

Eccentricity

e= Distance between foci  Distance between vertices =2(c)2(a)=(c)(a)

e2=(c)2(a)2=(a)2+(b)2(a)2=1+(b)2(a)2

(a)2e2=(a)2+(b)2

x2(a)2y2(a)2(e21)=1

Equation of hyperbola x2(a)2y2 (b)2=1

1. Centre

All chords passing through (a) point and bisected at that point is known as centre of hyperbola C(0,0)

2. Eccentricity

e= Distance between foci  Distance between vertices 

e=1+( conjugate axis )2( transverse axis )2 e=PSPM, where S is focus, P is any point on the hyperbola, PM is distance from directrix.

3. Foci

S and S are foci whose coordinates are S(ae,0) and S(ae,0)

4. Directrices

Z1Z1 and Z2Z2 are the directrices whose equations are x=(a)e and x=(a)e

5. Vertices

A and A are the vertices of hyperbola . A((a),0) and A((a),0).

6. Axes

The line AA’ is called transverse axis and the line perpendicular to its through the centre of the hyperbola is called conjugate axis. Equations of transverse axis is y=0 and equation of conjugate axis is x=0.

Length of transverse axis =2(a)

Length of conjugate axis=2 (b)

7. Double Ordinate

A chord of hyperbola which is perpendicular to transverse axis is known as double ordinate QQ,Q(h,k),Q(h,k).

8. Latus rectum

The double ordinates passing through focus is known as latus rectum.

L1(ae,(b)2(a)),L1(ae,(b)2(a))

L2(ae,(b)2(a)),L2(ae,(b)2(a))

9. Focal Chord

A chord passing through focus is known as focal chord.

10. Focal Distance

PS1=ePM1=e(x1(a)e)

=ex1(a)

PS2=e(x1+(a)e)

=ex1+(a)

PS2PS1=2(a)

Rectangular or Equilateral Hyperbola

If (a)=(b), then equation of hyperbola is

x2y2=(a)2 known as rectangular or equilateral hyperbola.

The eccentricity of rectangular hyperbola is

e2=1+(b)2(a)2=1+1

e=2

Equation of hyperbola if centre is ( h,k) and axes are parallel to coordinate axes is

(xh)2(a)2(yk)2 (b)2=1

Practice questions

1. The eccentricity of the conic 9x216y2=144 is

(a). 43

(b). 45

(c). 54

(d). 17

Show Answer Answer: (c)

2. The equation of the conic with focus at (1,1) directrix along xy+1=0 and eccentricity 2 is

(a). xy=1

(b). 2xy+4x4y1=0

(c). x2y2=1

(d). 2xy4x+4y+1=0

Show Answer Answer: (d)

3. Equation of the hyperbola whose vertices are (±3,0) and foci at (±5,0) is

(a). 9x225y2=144

(b). 16x29y2=144

(c). 9x216y2=144

(d). 25x29y2=225

Show Answer Answer: (b)

4. The eccentricity of the hyperbola whose latus rectum is half of its transverse axis, is

(a). 32

(b). 32

(c). 23

(d). 23

Show Answer Answer: (a)

5. The differences of the focal distances of any point on the hyperbola is equal to

(a). eccentricity

(b). latus-rectum

(c). length of conjugate axis

(d). length of transverse axis

Show Answer Answer: (d)

6. The distance between the foci of (a) hyperbola is 16 and its eccentricity is 2, then equation of the hyperbola is

(a). x2y2=32

(b). x2y2=16

(c). y2x2=16

(d). y2x2=32

Show Answer Answer: (a)

7. The length of latus rectum of the hyperbola 3x26y2=18 is

(a). 24

(b). 23

(c). 32

(d). 43

Show Answer Answer: (d)

8. The eccentricity of the hyperbola 9x216y218x+32y151=0 is

(a). 53

(b). 54

(c). 52

(d). 5

Show Answer Answer: (b)

9. The foci of (a) hyperbola coincide with the foci of the ellipse x225+y29=1. If its eccentricity is 2 then equation of hyperbola is

(a). x212y24=1

(b). x28y212=1

(c). x24y212=1

(d). x2y2=4

Show Answer Answer: (c)

10. The eccentricity of rectangular hyperbola is

(a). 2

(b). 1

(c). 3

(d). 2

Show Answer Answer: (d)