Hyperbola - Equation of Hyperbola (Lecture-01)

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.

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 ax+by+c=0 and whose eccentricity is e, is

(xh)2+(yk)2=e2(ax+by+c)2a2+b2

Eccentricity :

e= Distance between foci  Distance between vertices =2c2a=ca

e2=c2a2=a2+b2a2=1+b2a2

a2e2=a2+b2

x2a2y2a2(e21)=1

Equation of hyperbola x2a2y2b2=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 directries whose equation are x=ae and x=ae

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 =2a

Length of conjugate axis =2b

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,b2a),L1(ae,b2a)

L2(ae,b2a),L2(ae,b2a)

9.Focal chard : A chord passing through focus is known as focal chord.

10.Focal Distance :

PS1=ePM1=e(x1ae)

=ex1a

PS2=e(x1+ae)

=ex1+a

PS2PS1=2a

Rectangular or Equilateral Hyperbola

If a=b, then equation of hyperbola is x2y2=a2 known as rectangular or equilateral hyperbola. The eccentricity of rectangular hyperbola is

e2=1+b2a2=1+1

e=2

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

(xh)2a2(yk)2 b2=1

Position of a point :

Let a point P(x1,y1) and equations of hyperbola be x2a2y2 b21=0

S1=x12a2y12 b21

If S1>0, point is inside the hyperbola.

If S1=0, point is on the hyperbola.

If S1<0, point is outside the hyperbola.

Conjugate Hyperbola :

Corresponding to every hyperbola there exists a hyperbola such that the transverse axis and conjugate axis of one is equal to the conjugate axis and transverse axis of the other. Such hyperbolas are known as conjugate to each other.

Therefore for the hyperbola x2a2y2 b2=1

Conjugate hyperbola is x2a2y2 b2=1

Auxiliary circle and eccentric angle

A circle drawn with centre O and transverse axis as diameter is known as auxiliary circle. Equation of auxiliary circle is x2+y2=a2

A is any point on the circle whose coordinates are (acosθ, a sinθ ), where θ is known as eccentric angle.

Hyperbola Conjugate Hyperbola
Equation x2a2y2 b2=1 x2a2+y2 b2=1
Centre (0,0) (0,0)
Vertice (a,0) & (a,0) (0, b) & (0,b)
Foci (ae,0) & (ae,0) (o,be) & (0,be)
Length of transverse axis 2a 2 b
Length of conjugate axis 2 b 2a
Length of latus rectum 2 b2a 2a2 b
Equation of transverse axis y=0 x=0
Equation of conjugate axis x=0 y=0
Equation of directrices x=±ae y=±be
Eccentricity e=1+b2a2 or b2=a2(e21) e=1+a2 b2 or a2=b2(e21)

Equation of a Hyperbola referred to two perpendicular lines

Let equation of hyperbola be

x2a2y2 b2=1

From diagram PM=y and PN=x

PN2a2PM2 b2=1

y=±be

e=1+a2 b2 or a2=b2(e21)

ie. if perpendicular distance of a point P(x,y) from two mutually perpendicular lines say l1=a1x+b1y+c1=0 and l2=a2x+b2y+c2=0 then

(a1x+b1y+c1a12+b12)a2(a2x+b2y+c2a22+b22)b2=1

then the locus of point P denotes a hyperbola

  • centre of the hyperbola, we get after solving 1=0 and 2=0

  • Transverse axis : 2=0

  • Conjugate axis :1=0

  • Foci : The foci of the hyperbola is the point of intersection of the lines a1x+b1y+c1a12+b12=±ae and 2=0

  • Directrix: a1x+b1y+c1a12+b12=±ae

  • Length of transverse axis =2a

  • Length of conjugate axis =2b

  • Length of latus Rectum =2b2a

Equation of tangent

(i) Point Form T=0

Equation of tangent at point P(x1,y1) to the hyperbola (xh)2a2(yk)2 b2=1 is (xh)(x1h)a2(yk)(y1k)b2=1

(ii) Parametric Form :

Parametric equation of hyperbola is x=asecθ,y=btanθ

Equation of tangent is

xasecθybtanθ=1

(iii) Slope Form :

y=mx±a2m2b2

Hyperbola (x2a2y2b2=1) Point of contact.

Point form : xx1a2yy1 b2=1 (x1,y1)

Parametric Form : xasecθybtanθ=1 (asecθ,btanθ)

Slope form: y=mx±a2 m2b2

(±a2ma2m2b2,±b2a2m2b2)

Equation of Pair of Tangents

Let equation of hyperbola be x2a2y2 b2=1 and a point P(x1,y1) then the combined equation of

tangents PA and PB is SS1=T2 where

S=x2a2y2 b21 S1=x12a2y12 b21 T=xx1a2yy1 b2=1

Equation of Normal to the Hyperbola :

i. Point Form

x2a2y2b2=1

Slope to tangent =+b2x1a2y1

Slope to normal =a2y1b2x1

Equation of normal yy1=a2y1b2x1(xx1)

a2xx1+b2yy1=a2+b2

ii. Paramatric Form P(asecθ,btanθ)

axsecθ+bytanθ=a2+b2

or axcosθ+bycotθ=a2+b2

iii. Slope Form

y=mx±m(a2+b2)a2m2b2

Point of contanct is (±a2a2m2b2,mb2a2m2b2)

Equation of Chord of Contact

Let equation of hyperbola be x2a2y2 b2=1,

Equation of Chord of contact is

xx1a2yy1 b21=0

or T=0

where T=xx1a2yy1 b21

Equation of the chord of the hyperbola whose mid point is given :

A is the mid point of PQ, then equation of chord is

xx1a2yy1 b2=x12a2y12 b2

ie. T=S1 where

T=xx1a2yy1 b21

S1=x12a2y12 b21

Asymptotes of Hyperbola

An asymptotes of any hyperbola is a straight line which touches in it two points at infinity.

OR

If the length of the perpendicular let fall from a point on a hyperbola to a straight line tends to zero as the point on the hyperbola moves to infinity along the hyperbola, then the straight line is called asymptote of the hyperbola.

The equation of two anymptotes of the hyparbola x2a2y2b2=1

are y=±bax or xa±yb=0

Pair of asymptotoes : x2a2y2 b2=0

1. If b=a, then x2a2y2b2=1 reduces to x2y2=a2. The asymptotes of rectangular hyperbola x2y2=a2 are y=±x which are at right angles.

2. A hyperbola and its conjugate hyperbola have the same asyptotes.

3. The angle between the asymptotes of x2a2y2b2=1 is 2tan1(ba)

4. The asymptotes pass through the centre of the pyperbola.

5. The bisectors of the angle between the asymptotes are the coordinate axes.

6. Let H=x2a2y2 b21=0

A=x2a2y2b2=0

and C=x2a2y2 b2+1=0

be the equation of the hyperbola, asymptotes and the conjugate hyperbola respectively, then clearly

C+H=2 A

Rectangular Hyperbola:

A hyperbola whose asymptotes include a right angle is said to be rectangular hyperbola.

OR

If the lengths of transverse and conjugate axes of any hyperbola be equal it is called rectangular or equilateral hyperbola.

Then asymptotes of x2y2=a2 are x+y=0 and xy=0. Each of these two asymptotes is inclined at an angle of 45 with the transverse axis. So, if we rotate the coordinate axes through an angle of π4 keeping the origin fixed, then the axes coincide with the asymptotes of the hyperbola.

Now equation of asymptotes of new hyperbola is x=0

Then equation of hyperbola is xy=k (constant)

The hyperbola passes through the point (a2,a2)

k=a22

Then equation of hyperbola is xy=a22 or xy=c2 where c2=a22

If the asymptotes of a rectangular hyperbola are x=a,y=b, then its equation is (xa)(yb)=c2 xy=c2

1. Asymptotes : x=0,y=0

2. Transverse axis: y=x

Conjugate axis : y=x

3. Vertices A(c,c),A(c,c)

4. Foci : S(c2,c2),S(c2,c2)

5. Length of transverse axis =AA=22c

6. Equation of auxiliary circle x2+y2=2c2

7. Equation of director circle x2+y2=0

8. x2y2=a2 and xy=c2 intersect at right angles

Properties of Rectangular Hyperbola

1. Eccentricity of rectangular hyperbola is 2.

2. Since x=ct,y=ct satisfies xy=c2

(x,y)=(ct,ct)(t0) is called a ’ t ’ point on the rectangular hyperbola. The x=ct,y=ct represents its parametric equation with parameter ’ t '

3. Equation of chord joining P(Ct1,ct1) and Q(Ct2,ct2) is

x+yt1t2c(t1+t2)=0 Slope of chord =1t1t2

4. Equation of tangent at (x1,y1) is xy1+yx1=2c2

5. Equation of tangent at is xt+yt=2c

 Slope of tangent =1t2

6. Equation of normal at (x1,y1) is xx1yy1=x12y12

Equation of normal at is xt3yt2ct4+c=0

 Slope of normal =t2

7. Point of intersection of tangents at t1 and t2 is

(2ct1t2t1+t2,2ct1+t2)

8. Point of intersection of normal at t1 and t2 is

(ct1t2(t12+t1t2+t22)ct1t2(t1+t2),ct13t23+c(t12+t1t2+t22)t1t2(t1+t2))

Practice Problems

1. Equation of conjugate axis of hyperbola xy3y4x+7=0 is

(a) x+y=7

(b) x+y=3

(c) xy=7

(d) None of these

Show Answer

Solution:

xy3y4x+7=0xy3y4x+12=5(x3)(y4)=5

Equation of asymptotes are x3=0 and y4=0 Since the hyperbola is rectangular hyperbola, axes are bisectors of asymptotes

Hence their slaps are ±1

Equation of conjugate axis is

y4=1(x3)

x+y=7

Answer (A)

2. If S1 and S2 are the foci of the hyperbola whose transverse axis length is 4 and conjugate axis length is 6, S3 and S4 are the foci of the conjugate hyperbola, then the area of the quadrilateral S1 S3 S2 S4 is

(a) 156

(b) 36

(c) 26

(d) None of these

Show Answer

Solution:

S1 S3 S2 S4 forms a square.

So required area =4× area of ΔS1OS3=4×12 ae × be 1

=2abee1=2.2.3.ee1

=12ee1

Now e =1+94=132 & e1=1+94=132

Hence area =12×132×133=26 sq.units

Answer (C)

3. The ellipse 4x2+9y2=36 and the hyperbola a2x2y2=4 intersect at right angles then the equation of the circle through the points of intersection of two conic is

(a) x2+y2=25

(b) 5(x2+y2)+3x+4y=0

(c) 5(x2+y2)3x4y=0

(d) (x2+y2)=5

Show Answer

Solution:

Since ellipse and hyperbola intersect orthogonally, they are confocal.

e=149=53

foci of ellipse (±5,0)

(ae)2=a2+b25=4a2+4a=2

Let point of intersection in the first quadrant be P(x1,y1). P lies on both the curves.

4x12+9y12=36 and 4x12y12=4

Adding these two, we get 8x12+8y12=40

Equation of circle is x2+y2=5

x12+y12=5

4. If is the eccentricity of the hyperbola x2a2y2 b2=1 and 2θ is angle between the asymptotes then cosθ=

(a) 1e

(b) 1ee

(c) 1+ee

(d) None of these

Show Answer

Solution:

e=1+b2a2

we know 2θ=2tan1(ba)tanθ=ba

e=1+tanθ2=secθcosθ=1e

Answer (a)

5. From a point p(1,2) pair of tangents are drawn to a hyperbola in which one tangent to each arm of hyperbola. Equation of asymptotes of hyperbola are 3xy+5=0 and 3x+y1=0 then eccentricity of hyperbola is

(a) 3

(b) 23

(c) 2

(d) None of these

Show Answer

Solution:

Equation of asymptotes are

3xy+5=0

3xy+1=0

a1a2+b1 b2=3+1<0

origin lies in acute angle and P(1,2) lies in obtuse angle.

e=secθ where 2θ is the angle between asymptotes.

2θ=π3θ=π6

e=secπ6=23

Answer b

6. If a variable line has its intercepts on the coordinate axes e,e where e2,e2 are the eccentricities of a hyperbola and its conjugate hyperbola, then the line always touches the circle x2+y2=r2, where r=

(a) 4

(b) 3

(c) 2

(d) Can not be decided

Show Answer

Solution

Now 4e2+4(e)2=14=e2(e)2e2+(e)2

Line passing through the points (e,0) and (0,e) is ex+ey=e

It is a tangent to the circle x2+y2=r2

|eee2+(e)2|=r

2=r

Answer (c)

7. If angle between asymptotes of hyperbola x2a2y2b2=1 is 120 and product of perpendiculars drown from foci upon its any tangent is 9 , then locus of point of intersection of perpendicular tangents of the hyperbola can be

(a) x2+y2=18

(b) x2+y2=6

(c) x2+y2=9

(d) x2+y2=3

Show Answer

Solution

2.tan1ba=60ba=13 b2=9a2=27

Required locus is director circle i.e. x2+y2=279

x2+y2=18

If ba=tan60=3

a2=3

Then equation of director circle is x2+y2=39=6 which is not possible.

Answer (a)

8. The equation of the transverse axis of the hyperbola (x3)2+(y+1)2=(4x+3y)2 is

(a) 3x4y=0

(b) 4x+3y=0

(c) 3x4y=13

(d) 4x+3y=9

Show Answer

Solution

(x3)2+(y+1)2=(4x+3y)2(x3)2+(y+1)2=25(4x+3y5)2PS=5PM

Directrix is 4x+3y=0 and focus is (3,1)

Equation of transverse axis is y+1=34(x3)

3.x4y=13

Answer (c)

Exercise

1. The equation of common tangents to the parabola y2=8x and hyperbola 3x2y2=3 is

(a) x±2y1=0

(b) x±2y+1=0

(c) 2x±y+1=0

(d) 2x±y1=0

Show Answer Answer: c

2. A tangent to the hyperbola y=x+9x+5 passing through the origin is

(a) x2y=0

(b) 5xy=0

(c) 5x+y=0

(d) x+225y=0

Show Answer Answer: b

3. The equation of the common tangent to the curves y2=8x and xy=1 is

(a) y=x+2

(b) y=2x+1

(c) 2y=x+8

(d) 3y=9x+2

Show Answer Answer: a

4. Let PQ be a double ordinate of the hyperbola x2a2y2 b2=1. If O be the centre of the hyperbola and OPQ is an equilateral triangle, then eccentricity e is

(a) >3

(b) >2

(c) >23

(d) None of these

Show Answer Answer: c

5. The difference between the length 2a of the transverse axis of a hyperbola of eccentricity e and the length of its latus rectum is

(a) a(2e21)

(b) 2a(e21)

(c) 2a|3e2|

(d) 2a2e2

Show Answer Answer: d

6. The slopes of common tangents to the hyperbolas x29y216=1 and y29x216=1 are

(a) ±2

(b) ±2

(c) ±1

(d) None of these

Show Answer Answer: c

7. The two conics y2b2x2a2=1 and y2=bax intersect iff

(a) 0<b12

(b) 0<a12

(c) bPracticeProblems2<a2

(d) b2>a2

Show Answer Answer: a

8. The point on the hyperbola x224y218=1 which is nearest to the line 3x+2y+1=0 is

(a) (6,3)

(b) (3,6)

(c) (6,3)

(d) (6,3)

Show Answer Answer: d

9. If (asecθ,btanθ) and (asecϕ,btanϕ) be the coordinates of the ends of a focal chord of the hyperbola x2a2y2 b2=1, then tanθ2tanϕ2=

(a) 1+e1e

(b) 1e1+e

(c) e1e+1

(d) None of these

Show Answer Answer: b

10. If the latus rectum of a hyperbola through one focus subtends 60 angle at the other focus, then its eccentricity e is

(a) 2

(b) 3

(c) 5

(d) 6

Show Answer Answer: b