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Director circle and auxiliary circle of a hyperbola - class-XI

Description: director circle and auxiliary circle of a hyperbola
Number of Questions: 42
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Tags: maths mathematics and statistics two dimensional analytical geometry-ii hyperbola conic sections
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For the hyperbola $\dfrac{x^2}{64}-\dfrac{y^2}{36}=1$, the equation of director circle is 

  1. $x^2+y^2=100$

  2. $2x^2+2y^2=100$

  3. $x^2+y^2=28$

  4. $x^2-y^2=100$


Correct Option: C
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{64} -\dfrac {y^2}{36} = 1$,

Here $a =8$ and $b =6$

So equation of the director circle will be $x^2 + y^2 = (8)^2 - (6)^2$

$\Rightarrow x^2 + y^2 = 28$

Correct option is $C$.

The equation of auxillary circle of hyperbola is $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$

  1. $x^2+y^2=a^2$

  2. $x^2+y^2=2a^2$

  3. $x^2+y^2=a^2+b^2$

  4. $x^2+y^2=a^2-b^2$


Correct Option: A
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called the Auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

So the correct option is $A$

 The equation of director circle of $\dfrac{x^2}{64}-\dfrac{y^2}{49}=1$ is

  1. $x^2+y^2=15$

  2. $x^2+y^2=64$

  3. $x^2+y^2=18$

  4. $x^2+y^2=10$


Correct Option: A
Explanation:

Equation of hyperbola is $\dfrac { { x }^{ 2 } }{ 64 } -\dfrac { { y }^{ 2 } }{ 49 } =1$

Here $a=8,b=7$
Equation of director circle for hyperbola is
${ x }^{ 2 }+{ y }^{ 2 }={ a }^{ 2 }-{ b }^{ 2 }\ { x }^{ 2 }+{ y }^{ 2 }=64-49\ { x }^{ 2 }+{ y }^{ 2 }=15$
Hence, option A is correct.

The length of diameter of director circle of hyperbola $\dfrac{x^2}{49}-\dfrac{y^2}{25}=1$, is 

  1. $4$

  2. $6$

  3. $4\sqrt6$

  4. $24$


Correct Option: C
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{49} -\dfrac {y^2}{25} = 1$,

Here $a =7$ and $b =5$

So equation of the director circle will be $x^2 + y^2 = (7)^2 - (5)^2$

$\Rightarrow x^2 + y^2 = 24$

Hence the radius of the director circle is $\sqrt{24} = 2\sqrt{6}$. So the diameter will be $4\sqrt6$.

So correct option is $C$.

The equation of director circle for $\dfrac{x^2}{100}-\dfrac{y^2}{36}=1$, is

  1. $2x^2+2y^2=100$

  2. $\sqrt 2x^2+\sqrt 2y^2=100$

  3. $x^2+y^2=6$

  4. $x^2+y^2=64$


Correct Option: D
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{100} -\dfrac {y^2}{36} = 1$,

Here $a =10$ and $b =6$

So equation of the director circle will be $x^2 + y^2 = (10)^2 - (6)^2$

$\Rightarrow x^2 + y^2 = 64$

Correct option is $D$.

The equation of director circle of hyperbola $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$ is

  1. $x^2+y^2=a^2$

  2. $x^2+y^2=b^2$

  3. $x^2+y^2=a^2+b^2$

  4. $x^2+y^2=a^2-b^2$


Correct Option: D
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Hence the Director circle is a circle whose centre is same as centre of the hyperbola and the radius is $\sqrt{a^2 - b^2}$

So correct option is $D$.

The circle passing through the vertices of hyperbola is called 

  1. director circle

  2. auxillary circle

  3. nine point circle

  4. none


Correct Option: B
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

As center of the Auxiliary circle is same as the center of the  hyperbola i.e. origin and the diameter is $2a$, hence the circle touches the two vertices of hyperbola $(a,0)$ and $(-a,0)$

Hence we can say that the circle passing through the two vertices of the hyperbola is Auxiliary circle. 
So the correct option is $B$.

The intersection point of,a perpendicular on tangent of a hyperbola from the focus  and a tangent lies on 

  1. director circle

  2. auxillary circle

  3. nine point circle

  4. none


Correct Option: B
Explanation:
The intersection point of a perpendicular on tangent of a hyperbola from the focus and a tangent lies on: Auxiliary circle
Auxiliary circle of a hyperbola which is a circle described on the major axis of a hyperbola as its diameter.
Let the hyperbola be
$\cfrac { { x }^{ 2 } }{ { a }^{ 2 } } -\cfrac { { y }^{ 2 } }{ { b }^{ 2 } } =1$
The equation of auxiliary circle is $x^2+y^2=a^2$
Take a point $P(x _1,y _1)$
Through $P$ draw a line perpendicular to major axis intersecting major axis in $N$ and auxiliary circle in $P'$.
The points $P$ and $P'$ are called as corresponding points on the hyperbola and auxiliary circle respectively.
This angle is known as the eccentric angle of the point $P$ on the hyperbola and auxiliary circle respectively.

If $\theta$ is eliminated from the equations $a\sec\theta - x\tan\theta = y \mbox{ and } b\sec\theta + y\tan\theta = x$ ($a$ and $b$ are constant), then the eliminant denotes the equation of 

  1. the director circle of the hyperbola $\displaystyle\frac{x^2}{a^2} - \displaystyle\frac{y^2}{b^2} = 1$

  2. auxiliary circle of the ellipse $\displaystyle\frac{x^2}{a^2} + \displaystyle\frac{y^2}{b^2} = 1$

  3. director circle of the ellipse $\displaystyle\frac{x^2}{a^2} + \displaystyle\frac{y^2}{b^2} = 1$

  4. director circle of the circle $x^2 + y^2 = \displaystyle\frac{a^2 + b^2}{2}$


Correct Option: C
Explanation:

Solving given equation we get,
$\displaystyle \tan\theta=\frac{ax-by}{ay+bx}$
and $\displaystyle \sec\theta = \frac{x^2+y^2}{ay+bx}$
Eliminating $\theta$ we get, $x^2+y^2=a^2+b^2$ which is director  circle of the ellipse $\displaystyle\frac{x^2}{a^2} + \displaystyle\frac{y^2}{b^2} = 1$

If pair of tangents are drawn from any point $(p)$ on the circle ${x^2} + {y^2} = 1$ to the hyperbola $\frac{{{x^2}}}{2} - \frac{{{y^2}}}{1} = 1$ such that locus of circumcenter of triangle formed by pair of tangents and chord of contact is ${\lambda _1}{x^2} - 2{\lambda _2}{y^2} = 2{\left( {\frac{{{x^2}}}{2} - {y^2}} \right)^2}$, then 

  1. ${\lambda _1} = 2,{\lambda _2} = 1$

  2. ${\lambda ^2} _1 + {\lambda ^2} _2 = 5$

  3. ${\lambda _1} = 1,{\lambda _2} = - 1$

  4. ${\lambda ^2} _1 + {\lambda ^2} _2 = 2$


Correct Option: A

Find the range of $p$ such that no perpendicular tangents can be drawn to the hyperbola $\dfrac{x^2}{(-p^2 + 6p + 5)} - \dfrac{y^2}{(-p - 3)} = 1$, i.e. the director circle of the given hyperbola is imaginary.

  1. $R - [-1 , 8]$

  2. $(5 , 6)$

  3. $(3 , 4)$

  4. $(-7 , 4)$


Correct Option: A
Explanation:

The director circle is the locus of the point of intersection of a pair of perpendicular tangents to a hyperbola.

Equation of the director circle of the hyperbola $\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1$ is $x^2 + y^2 = a^2  b^2$ i.e. a circle whose center is origin and radius is $(a^2  b^2)$.

Hence, for the director circle to be imaginary, $a^2 < b^2$ .

$-p^2 + 6p + 5 < -p - 3$ ---> $p^2 -7p -8 > 0$ - $(p-  8)(p + 1) > 0$ $\Rightarrow$ $p$ lies in $R - [-1 , 8]$. Hence, option (A).

For the hyperbola $\dfrac{x^2}{49}-\dfrac{y^2}{25}=1$, the equation of auxillary circle is

  1. $x^2+y^2=49$

  2. $x^2+y^2=25$

  3. $x^2+y^2=10$

  4. $x^2+y^2=10074$


Correct Option: A
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called the Auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

Here the given hyperbola is $\dfrac{x^2}{49}-\dfrac{y^2}{25}=1$, which is similar to standard form of the hyperbola.

Here $a = 7$ and $b = 5$

The equation of Auxiliary circle for the given hyperbola will also be $x^2 + y^2 = (7)^2$

$\Rightarrow x^2 + y^2 = 49$

So the correct option is $A$

The radius of the director circle of the ellipse $9{x^2} + 25{y^2} - 18x - 100y - 116 = 0$ is 

  1. $\sqrt {34} \,$

  2. $\sqrt {29} \,\,\,$

  3. 5

  4. 8


Correct Option: A

The  equation of  auxillary circle of  $\dfrac{x^2}{64}-\dfrac{y^2}{36}=1$ is

  1. $x^2+y^2=100$

  2. $x^2+y^2=50$

  3. $x^2+y^2=64$

  4. $x^2+y^2=36$


Correct Option: C
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called the Auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

Here the given hyperbola is $\dfrac{x^2}{64}-\dfrac{y^2}{36}=1$, which is similar to standard form of the hyperbola.

Here $a = 8$ and $b = 6$

The equation of Auxiliary circle for the given hyperbola will also be $x^2 + y^2 = (8)^2$

$\Rightarrow x^2 + y^2 = 64$

So the correct option is $C$

For the hyperbola $\dfrac{x^2}{15}-\dfrac{y^2}{10}=1$, the equation of auxillary circle is

  1. $x^2+y^2=15$

  2. $x^2+y^2=10$

  3. $x^2+y^2=35$

  4. $x^2+y^2=5$


Correct Option: A
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called the Auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

Here the given hyperbola is $\dfrac{x^2}{15}-\dfrac{y^2}{10}=1$, which is similar to standard form of the hyperbola.

Here $a = \sqrt{15}$ and $b = \sqrt{10}$

The equation of Auxiliary circle for the given hyperbola will also be $x^2 + y^2 = (\sqrt{15})^2$

$\Rightarrow x^2 + y^2 = 15$

So the correct option is $A$

If ${e _1}$and ${e _2}$ are the eccentricities of the hyperbolas $xy = 9$ and ${x^2} - {y^2} = 25$ ,then( ${e _1}$,${e _2}$) lie on a circle ${C _1}$with centre origin then the ${(radius)^2}$ of the director circle of ${C _1}$is

  1. $2$

  2. $4$

  3. $8$

  4. none


Correct Option: A

 The equation of auxillary circle is $\dfrac{x^2}{25}-\dfrac{y^2}{16}=1$

  1. $x^2+y^2=16$

  2. $x^2+y^2=32$

  3. $x^2+y^2=25$

  4. $x^2+y^2=41$


Correct Option: C
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$,


The circle drawn taken major axis as a diameter also called the Auxiliary circle of the Hyperbola, will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any standard Hyperbola will be $x^2+y^2=a^2$

Here the given hyperbola is $\dfrac{x^2}{25}-\dfrac{y^2}{16}=1$, which is similar to standard form of the hyperbola.

Here $a = 5$ and $b = 4$

The equation of Auxiliary circle for the given hyperbola will also be $x^2 + y^2 = (5)^2$

$\Rightarrow x^2 + y^2 = 25$

So the correct option is $C$

If the chords of contact of tangents drawn from $P$ to the hyperbola $x^2 - y^2 = a^2$ and its auxiliary circle are at right angle, then $P$ lies on :

  1. $x^2 - y^2 = 3a^2$

  2. $x^2 - y^2 = 2a^2$

  3. $x^2 - y^2 = 0$

  4. $x^2 - y^2 = 1$


Correct Option: C
Explanation:

Let $P$ be $(h,k)$
Now Chord of contact of tangent from $P$ to the hyperbola $x^2-y^2=a^2$ is,
$T =0\Rightarrow hx -ky = a^2$ (i)
And director circle of given hyperbola is, $x^2+y^2=a^2$
Thus equation of chord of contact to this circle from P is, $hx+ky = a^2$ (ii)
Now given line (i) and (ii) are perpendicular,
$\Rightarrow \cfrac{h}{k}\times \cfrac{-h}{k}=-1\Rightarrow h^2=k^2$
Hence locus of $P$ is given by, $x^2-y^2=0$ 

If the circle $x^2\, +\, y^2\, =\, a^2$ intersects the hyperbola $xy\, =\, c^2$ in four points $P\, (x _1,\, y _1),\, Q(x _2,\, y _2),\, R(x _3,\, y _3),\, S(x _4,\, y _4)$, then -

  1. $X _1\, +\, X _2\, +\, X _3\, +\, X _4\, =\, 0$

  2. $Y _1\, +\, Y _2\, +\, Y _3\, +\, Y _4\, =\,0$

  3. $X _1\, X _2\, X _3\, X _4\, =\, c^4$

  4. $Y _1\,Y _2\,Y _3\,Y _4\, =\, c^4$


Correct Option: A,B,C,D
Explanation:

Since, the circle $x^2\, +\, y^2\, =\, a^2$ intersects the hyperbola $xy\, =\, c^2$
Therefore, $x^2+\dfrac{c^4}{x^2}=a^2$
$\Rightarrow x^4-a^2x^2+c^4=0$
now sum of the roots: $x _1+x _2+x _3+x _4=0$
and product of the roots $x _1x _2x _3x _4=c^4$
Repeat the same for $y$

The radius of the director circle of the hyperbola $\dfrac{x^{2}}{a^{2}}-\dfrac{y^{2}}{b^{2}}=1$ is 

  1. $a-b$

  2. $\sqrt{a-b}$

  3. $\sqrt{a^{2}-b^{2}}$

  4. $\sqrt{a^{2}+b^{2}}$


Correct Option: A

If one of the directrix of hyperbola $\dfrac{x^2}{9}-\dfrac{y^2}{b}=1$ is $x=-\dfrac{9}{5}$. Then the corresponding focus of hyperbola is?

  1. $(5, 0)$

  2. $(-5, 0)$

  3. $(0, 4)$

  4. $(0, -4)$


Correct Option: B
Explanation:

Given hyperbola is $\dfrac{x^2}{9}-\dfrac{y^2}{b}=1$

Directrix is $x=-\dfrac{9}{5}\implies \dfrac{3}{e}=\dfrac{9}{5}\implies e=\dfrac{5}{3}$
Corresponding focus of hyperbola is $(-a e,0)=(-5,0)$

The equation of the director circle of the hyperbola $\dfrac{x^2}{81}- \dfrac{y^2}{16}=1$ is

  1. $x^2+y^2=65$

  2. $x^2+y^2=97$

  3. $(x-9)^2+(y-4)^2=0$

  4. $(x+9)^2+(y+4)^2=0$


Correct Option: A
Explanation:

for hyperbola,

$\dfrac{x^2}{81}- \dfrac{y^2}{16}=1$ 
equation of director circle is 
$x^2+y^2=a^2-b^2$
$x^2+y^2=81-16=65$

The equation of the director circle of the hyperbola $\dfrac{x^2}{36}- \dfrac{y^2}{16}=1$ is

  1. $x^2+y^2=20$

  2. $x^2+y^2=52$

  3. $(x-9)^2+(y-4)^2=0$

  4. $(x+9)^2+(y+4)^2=0$


Correct Option: A
Explanation:

for hyperbola, $\dfrac{x^2}{36}- \dfrac{y^2}{16}=1$ 

equation of director circle is $x^2+y^2=a^2-b^2=36-16=20$

Auxiliary circle of a hyperbola is defined as:

  1. The auxiliary circle for a hyperbola is a circle with its centre on the polar and contains the two vertices.

  2. The circle whose center concurs with that of the ellipse and whose radius is equal to the ellipse's semimajor axis.

  3. The auxiliary circle for a hyperbola is a circle with its centre on the axis and contains the two vertices.

  4. None of these


Correct Option: C
Explanation:

The Auxiliary circle of a hyperbola is defined as the circle with the center same as the hyperbola and with transverse axis as it's diameter. The end points of transverse axis are the two vertices of the hyperbola, so the circle also contains the two vertices of the hyperbola.


The equation of Auxiliary circle foe a hyperbola is given by $x^2  + y^2 = a^2$

Hence the correct answer is Option $C$.

The circle with major axis as diameter is called the auxiliary circle of the hyperbola. 
If $a>b,$ then the equation of auxiliary circle is

  1. $x^2+y^2=a^2$

  2. $x^2+y^2=b^2$

  3. $x^2+y^2=a^2-b^2$

  4. $x^2+y^2=a^2+b^2$


Correct Option: A
Explanation:

For any Hyperbola of the form $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$


Length of Major axis is always $2a$, though $a$ can be greater, lesser or equal to $b$

For any conjugate Hyperbola $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=-1$ or $\dfrac{y^2}{b^2}-\dfrac{x^2}{a^2}=1$

The length of major axis is always $2b$, Though $b$  can be greater, lesser or equal to $a$

So For any Hyperbola the circle drawn taken major axis as a diameter also called auxiliary circle of the Hyperbola will have a diameter of $2a$, equal to the length of major axis and center same as center of Hyperbola.

Hence equation of Auxiliary circle of any Hyperbola will be $x^2+y^2=a^2$

So the correct option is $A$

The equation of director circle of the hyperbola $-\dfrac{x^2}{a^2}+ \dfrac{y^2}{b^2}=1$, if $b>a$,  is 

  1. $x^2+y^2=b^2-a^2$

  2. $x^2+y^2=b^2$

  3. $x^2+y^2=a^2$

  4. $x^2+y^2=b^2+a^2$


Correct Option: A
Explanation:

The equation of director circle whose center us at origin and radius $\sqrt{a^2-b^2}$

So, $x^2+y^2=a^2-b^2$
If $b>a$, then the equation of director circle of the hyperbola will be  $x^2+y^2=b^2-a^2$, where radius will be $\sqrt{b^2-a^2}$.

The radius of director circle of the hyperbola $\dfrac{x^2}{16}-\dfrac{y^2}{9}=1$ is

  1. $6$

  2. $7$

  3. $\sqrt 7$

  4. $8$


Correct Option: C
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{16} -\dfrac {y^2}{9} = 1$,

Here $a =4$ and $b =3$

So equation of the director circle will be $x^2 + y^2 = (4)^2 - (3)^2$

$\rightarrow x^2 + y^2 = 7$

Hence the radius of the director circle is $\sqrt7$. So correct option is $C$.

The equation of director circle of $-\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1$, If $b<a$ is:

  1. $x^2+y^2=b^2-a^2$

  2. $x^2+y^2=b^2+a^2$

  3. $x^2-y^2=b^2-a^2$

  4. Director circle does not exist


Correct Option: D
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$

The given hyperbola $-\dfrac{x^2}{a^2} +\dfrac {y^2}{b^2} = 1$ is a conjugate hyperbola. 

Hence the equation of Director circle for a conjugate hyperbola is given by $x^2 + y^2 = b^2 - a^2$


Hence the Director circle of given hyperbola is a circle whose center is same as center of the given hyperbola and the radius is $\sqrt{b^2 - a^2}$

As the radius is always a positive and real value, so $(b^2 - a^2) > 0$

$\rightarrow (b-a)(b+a) > 0$

As $a$ and $b$ both are positive quantities hence $a + b >0$

Hence $b-a > 0$  or  $b > a$

For $b < a$ the director circle does not exist, as the radius will not be real for $b<a$

So correct option is $D$.

The equation of the auxiliary circle of the hyperbola $4x^2-9y^2=36$ is

  1. $x^2+y^2=81$

  2. $x^2+y^2=9$

  3. $x^2+y^2=16$

  4. $x^2+y^2=4$


Correct Option: B
Explanation:

for hyperbola $a > b$

equation of auxiliary circle is $x^2+y^2=a^2$
$x^2+y^2=9$ is the equation of auxiliary circle for the given hyperbola

If any tangent to the hyperbola  $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$ with centre $C$, meets its director circle in $P$ and $Q$, then:

  1. $CP$ and $CQ$ are perpendicular to each other.

  2. $CP$ and $CQ$ are conjugate semi-diameters of the hyperbola.

  3. $CP$ and $CQ$ are not conjugate semi-diameters of the hyperbola.

  4. None of the above


Correct Option: B

The radius of  director circle of hyperbola is $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$

  1. $a$

  2. $b$

  3. $\sqrt{a^2+b^2}$

  4. $\sqrt{a^2-b^2}$


Correct Option: D
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Hence the Director circle is a circle whose centre is same as centre of the hyperbola and the radius is $\sqrt{a^2 - b^2}$

So correct option is $D$.

The equation of director circle of $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$ is:

  1. Imaginary if $a < b$

  2. Imaginary if $a>b$

  3. Point circle if $a=b$

  4. None of the above


Correct Option: A,C
Explanation:
The director circle of an hyperbola circumscribes the minimum bounding box of the hyperbola.it has the same center as the hyperbola, with radius $\sqrt { { a }^{ 2 }-{ b }^{ 2 } } $ where $a$ and $b$ are the semi-major axis and semi-minor axis of the hyperbola.
equation of circle is 
$x^2+y^2=a^2-b^2$
$a<b$ then circle radius is negative and circle is imaginary 
$a=b$ then circle radius is zero and circle is point circle

The director circle intersects its hyperbola in _______ number of points.

  1. zero

  2. two

  3. three

  4. four


Correct Option: A
Explanation:

For a standard hyperbola $\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1$, the equation of director circle is given by:


$x^2 + y^2 = a^2-b^2$

So we can see the center of the director circle of the standard hyperbola is same as the hyperbola. the radius of director circle is $\sqrt{a^2-b^2}$

As $a^2>0$, $a^2 > a^2 -b^2$

or $a > \sqrt{a^2-b^2}$

So we can see the radius of director circle is less the vertex of the hyperbola. Hence with a center same as the hyperbola, the director circle doesn't cut the hyperbola at any point. or we can say it's just smaller than the Auxiliary circle of the hyperbola which touches the vertices of the hyperbola. 

Hence the correct option is $A$.

The radius of the director circle of the hyperbola $\dfrac{x^2}{a(a+4b)}-\dfrac{y^2}{b(2a-b)}=1; 2a > b > 0$ is: 

  1. $a^2+b^2+4ab$

  2. $a+b$

  3. $a^2+b^2+2ab$

  4. $2(a+b)^2$


Correct Option: B
Explanation:

$\begin{array}{l} equation\, \, of\, \, director\, \, circle\, \, { x^{ 2 } }+{ y^{ 2 } }={ a^{ 2 } }+4ab-\left( { 2ab-{ b^{ 2 } } } \right)  \ \Rightarrow { x^{ 2 } }+{ y^{ 2 } }={ a^{ 2 } }+{ b^{ 2 } }+2ab \ \Rightarrow { x^{ 2 } }+{ y^{ 2 } }={ \left( { a+b } \right) ^{ 2 } } \ Then,\, \, Radians\, \, of\, \, circle\, \, is\left( { a+b } \right)  \end{array}$

The diametre of director circle of hyperbola $\dfrac{x^2}{25}-\dfrac{y^2}{16}=1$ 

  1. $3$

  2. $9$

  3. $6$

  4. $2$


Correct Option: C
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{25} -\dfrac {y^2}{16} = 1$,

Here $a =5$ and $b =4$

So equation of the director circle will be $x^2 + y^2 = (5)^2 - (4)^2$

$\rightarrow x^2 + y^2 = 9$

Hence the radius of the director circle is $\sqrt9 = 3$. So the diameter will be $6$.

So correct option is $C$.

 The  equation of director circle of hyperbola is $\dfrac{x^2}{36}-\dfrac{y^2}{25}=1$ is

  1. $x^2+y^2=4$

  2. $x^2+y^2=11$

  3. $x^2-y^2=4$

  4. $x^2+y^2=61$


Correct Option: B
Explanation:

The Director circle of a hyperbola is defined as the locus of the point of intersection of two perpendicular tangents to the hyperbola. For any standard hyperbola $\dfrac{x^2}{a^2} -\dfrac {y^2}{b^2} = 1$,


The equation of Director circle is given by $x^2 + y^2 = a^2 - b^2$


Here the given hyperbola is $\dfrac{x^2}{36} -\dfrac {y^2}{25} = 1$,

Here $a =6$ and $b =5$

So equation of the director circle will be $x^2 + y^2 = (6)^2 - (5)^2$

$\Rightarrow x^2 + y^2 = 11$

Correct option is $B$.

Point P is on the orthogonal hyperbola $x^2 - y^2 = a^2$. Point P' is the perpendicular projection of P on the x-axis. Then, $|PP'|^2$ is equal to the power of point P' relative to which circle?

  1. $x^2 + y^2 = a^2$

  2. $x^2 + y^2 = a^2 + b^2$

  3. Director circle

  4. Auxiliary circle


Correct Option: A,D
Explanation:

$P(a sec(t) , a tan(t))$ and $P'(a sec(t) , 0)$
$|PP'| = a^2 tan^2(t)$
The power of point P' relative to a circle $x^2 + y^2 = a^2$ is :

$(asec(t) - 0)^2 + (0-0)^2 - a^2$   (power of the point w.r.t. circle)

$= a^2sec^2(t) - a^2 = a^2(sec^2(t)-1) = a^2tan^2(t)$

Hence, the correct options are A and D.

The pole of the line $lx + my + n = 0$ with respect to the hyperbola $\displaystyle \frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$, is

  1. $\displaystyle \left ( \frac{a^2 l}{n} , \frac{b^2 m}{n} \right )$

  2. $\displaystyle \left ( - \frac{a^2 l}{n} , \frac{b^2 m}{n} \right )$

  3. $\displaystyle \left ( \frac{a^2 l}{n} , -\frac{b^2 m}{n} \right )$

  4. $\displaystyle \left ( -\frac{a^2 l}{n} , -\frac{b^2 m}{n} \right )$


Correct Option: B
Explanation:

Let $P\left( { x } _{ 1 },{ y } _{ 1 } \right) $ be the pole of the line

$lx+my+n=0$ with respect ot the hyperbola $\cfrac { { x }^{ 2 } }{ {

a }^{ 2 } } -\cfrac { { y }^{ 2 } }{ { b }^{ 2 } } =1$
Then the equation of the polar is
$\cfrac

{ { xx } _{ 1 } }{ { a }^{ 2 } } -\cfrac { { yy } _{ 1 } }{ { b }^{ 2 } }

=1\cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot (i)$
Since $\left( { x } _{ 1 },{ y } _{ 1 } \right) $  is the pole of the line
$lx+my+n=0\cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot (ii)$
Clearly $(i)$ and $(ii)$ represent the same line. Therefore,
$\therefore \quad \cfrac { { x } _{ 1 } }{ { a }^{ 2 }l } =\cfrac { { -y } _{ 1 } }{ { b }^{ 2 }m } =\cfrac { 1 }{ -n } $
${ x } _{ 1 }=\cfrac { { -a }^{ 2 }l }{ n } ,\quad { y } _{ 1 }=\cfrac { { b }^{ 2 }m }{ n } $
Hence the pole of the given line with respect of the given hyperbola is
$\left(- \cfrac { { a }^{ 2 }l }{ n } ,\cfrac { { b }^{ 2 }m }{ n }  \right) \quad $

The number of points from where a pair of perpendicular tangents can be drawn to the hyperbola, $ x^2 \sec^2\alpha-y^2 \cos ec^2\alpha=1, \alpha\in(0,\dfrac{\pi}4) $ are

  1. $0$

  2. $1$

  3. $2$

  4. infinite


Correct Option: D
Explanation:

The tangent equation to the hyperbola $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$ is 
$y=mx \pm \sqrt{a^2m^2-b^2}$
$\Rightarrow (y-mx)=\pm \sqrt{a^2m^2-b^2}$
On squaring both sides, we get
$\Rightarrow (y-mx)^2=(a^2m^2-b^2)$
$\Rightarrow (x^2-a^2)m^2-2xym+(y^2+b^2)=0$
Product of the slopes,$m _1m _2=\dfrac{(y^2+b^2)}{x^2-a^2}$
But given tangents are perpendicular to each other $\Rightarrow$ Their product of slopes equal to $-1.$
$\Rightarrow m _1m _2=-1$
$\Rightarrow \dfrac{(y^2+b^2)}{x^2-a^2}=-1$
$\Rightarrow x^2+y^2=(a^2-b^2)$
But given hyperbola equation as $\dfrac{x^2}{\cos\alpha^2}-\dfrac{y^2}{\sin\alpha^2}=1$
The required tangent equation is $x^2+y^2=\cos^2\alpha-\sin^2\alpha=\cos 2\alpha$
Since radius of circle is always greater than equal to zero.
$\Rightarrow \cos 2\alpha \geq 0$
But maximum value of $\cos$ is $1$.
$\Rightarrow 0 \leq \cos2\alpha \leq 1$
$\Rightarrow  \dfrac{\pi}{2} \leq 2\alpha \leq 0$
$\Rightarrow  \dfrac{\pi}{4} \leq \alpha \leq 0$
$\Rightarrow \alpha$has inifinite number of solutions.

The locus of the point of intersection of two perpendicular tangents to the hyperbola $\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1$ is

  1. Director circle

  2. $x^2 + y^2 = a^2$

  3. $x^2 + y^2 = a^2 - b^2$

  4. $x^2 + y^2 = a^2 + b^2$


Correct Option: A,C
Explanation:

Equation of any tangent in terms of slope $m$ is

$y = mx + (a^2m^2  b^2)$

It passes through $(h, k)$, so we have

$(k - mh)^2 = a^2m^2 - b^2$

So, $m^2(h^2 - a^2) - 2mhk + k^2 + b^2 = 0$

This is a quadratic in $m$

Let the slopes of tangents be $m _1$ and $m _2$.

then $m _1.m _2 = -1$.

So, $\dfrac{(k^2 + b^2)}{(h^2 - a^2)} = -1$

$(h^2 + k^2) = (a^2  b^2)$

Hence, the locus is $(x^2 + y^2) = (a^2  b^2)$ which is the director circle of $\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1.$

If the tangent at the point $(h, k)$ to the hyperbola $\dfrac{x^2}{a^2}\, -\, \dfrac{y^2}{b^2}\, =\, 1$ cuts the auxiliary circle in points whose ordinates are $y _1$ and $ y _2$, then  $\dfrac{1}{y _1} + \dfrac{1}{y _2} =$.

  1. $\dfrac{4}{k}$

  2. $\dfrac{3}{k}$

  3. $\dfrac{2}{k}$

  4. None of these


Correct Option: C
Explanation:

Equation of tangent of given hyperbola at point
$\displaystyle (h,\, k)\,$ is $\dfrac{hx}{a^2}\, -\, \dfrac{ky}{b^2}\, =\, 1$ ...(i)
Equation of auxillary circle is $x^2\, +\, y^2\, =\, a^2$ .....(ii)
From (i) and (ii)
$\displaystyle \left [ \left ( 1\, +\, \frac{ky}{b^2}\right ) \frac{a^2}{h}\right ]^2\, +\, y^2\, -\, a^2\, =\, 0$
$\Rightarrow\, y^2\, (k^2a^4\, +\, b^4h^2)\, +\, 2kb^2a^4y\, +\, b^4a^2\, (a^2\, -\, h^2)\, =\, 0$
Now  $\displaystyle \, \frac{y _1\, +\, y _2}{y _1y _2}\, =\, -\,

\frac{2kb^2a^4}{b^4a^2(a^2\, -\, h^2)}\, =\, \frac{-2ka^2}{b^2a^2 \left (

1\, -\, \frac{h^2}{a^2}\right )}$
$\displaystyle =\, \frac{-2k}{b^2 \left ( \frac{-k^2}{b^2}\right )}\, =\, \frac{2}{k}$

Find the range of $p$ such that a unique pair of perpendicular tangents can be drawn to the hyperbola $\dfrac{x^2}{(p^2 - 4)} - \dfrac{y^2}{(p^2 + 4p + 3)} = 1$, i.e. the director circle of the given hyperbola is a point.

  1. $p > 2$

  2. $p = {-\dfrac{7}{4}}$

  3. $p < -2$

  4. $p = {3}$


Correct Option: B
Explanation:

The director circle is the locus of the point of intersection of a pair of perpendicular tangents to a hyperbola.

Equation of the director circle of the hyperbola $\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1$ is $x^2 + y^2 = a^2  b^2$ i.e. a circle whose center is origin and radius is $(a^2  b^2)$.

Hence, for the director circle to be a point circle, $a^2 = b^2$ .

$p^2 - 4 = p^2 + 4p + 3$ ---> $4p = -7$ ---> $p = -\dfrac{7}{4}$ . Hence, option (B).

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