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Hyperbola Questions in English

Class 11 Mathematics · 10-2. Parabola, Ellipse, Hyperbola · Hyperbola

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251
DifficultMCQ
If a hyperbola passes through the point $P(10, 16)$ and it has vertices at $(\pm 6, 0)$,then the equation of the normal to it at $P$ is
A
$x + 2y = 42$
B
$3x + 4y = 94$
C
$2x + 5y = 100$
D
$x + 3y = 58$

Solution

(C) The standard equation of a hyperbola with vertices at $(\pm a, 0)$ is $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$.
Given $a = 6$,the equation is $\frac{x^2}{36} - \frac{y^2}{b^2} = 1$.
Since the hyperbola passes through $P(10, 16)$,we have $\frac{100}{36} - \frac{256}{b^2} = 1$.
$\frac{25}{9} - 1 = \frac{256}{b^2} \implies \frac{16}{9} = \frac{256}{b^2} \implies b^2 = \frac{256 \times 9}{16} = 144$.
The equation of the hyperbola is $\frac{x^2}{36} - \frac{y^2}{144} = 1$.
The equation of the normal at $(x_1, y_1)$ to the hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ is given by $\frac{a^2 x}{x_1} + \frac{b^2 y}{y_1} = a^2 + b^2$.
Substituting $a^2 = 36, b^2 = 144, x_1 = 10, y_1 = 16$:
$\frac{36x}{10} + \frac{144y}{16} = 36 + 144 = 180$.
$3.6x + 9y = 180$.
Dividing by $1.8$,we get $2x + 5y = 100$.
252
DifficultMCQ
Find the equations of the tangent and normal to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ at the point $(x_{0}, y_{0})$.
A
Tangent: $\frac{x x_{0}}{a^{2}}-\frac{y y_{0}}{b^{2}}=1$,Normal: $\frac{y-y_{0}}{a^{2} y_{0}}+\frac{x-x_{0}}{b^{2} x_{0}}=0$
B
Tangent: $\frac{x x_{0}}{a^{2}}+\frac{y y_{0}}{b^{2}}=1$,Normal: $\frac{y-y_{0}}{a^{2} y_{0}}-\frac{x-x_{0}}{b^{2} x_{0}}=0$
C
Tangent: $\frac{x x_{0}}{a^{2}}-\frac{y y_{0}}{b^{2}}=1$,Normal: $\frac{y-y_{0}}{a^{2} y_{0}}-\frac{x-x_{0}}{b^{2} x_{0}}=0$
D
Tangent: $\frac{x x_{0}}{a^{2}}-\frac{y y_{0}}{b^{2}}=1$,Normal: $\frac{y-y_{0}}{a^{2} y_{0}}+\frac{x-x_{0}}{b^{2} x_{0}}=0$

Solution

(D) Differentiating $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ with respect to $x$,we get $\frac{2x}{a^{2}}-\frac{2y}{b^{2}}\frac{dy}{dx}=0$.
Thus,$\frac{dy}{dx}=\frac{b^{2}x}{a^{2}y}$.
The slope of the tangent at $(x_{0}, y_{0})$ is $m_{t} = \frac{b^{2}x_{0}}{a^{2}y_{0}}$.
The equation of the tangent is $y-y_{0} = \frac{b^{2}x_{0}}{a^{2}y_{0}}(x-x_{0})$,which simplifies to $\frac{x x_{0}}{a^{2}}-\frac{y y_{0}}{b^{2}}=1$.
The slope of the normal is $m_{n} = -\frac{1}{m_{t}} = -\frac{a^{2}y_{0}}{b^{2}x_{0}}$.
The equation of the normal is $y-y_{0} = -\frac{a^{2}y_{0}}{b^{2}x_{0}}(x-x_{0})$,which can be written as $\frac{y-y_{0}}{a^{2}y_{0}} + \frac{x-x_{0}}{b^{2}x_{0}} = 0$.
253
DifficultMCQ
The locus of the midpoints of the chord of the circle $x^{2}+y^{2}=25$ which is tangent to the hyperbola $\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$ is
A
$\left(x^{2}+y^{2}\right)^{2}-16x^{2}+9y^{2}=0$
B
$\left(x^{2}+y^{2}\right)^{2}-9x^{2}+144y^{2}=0$
C
$\left(x^{2}+y^{2}\right)^{2}-9x^{2}-16y^{2}=0$
D
$\left(x^{2}+y^{2}\right)^{2}-9x^{2}+16y^{2}=0$

Solution

(D) Let the midpoint of the chord be $(h, k)$.
The equation of the chord of the circle $x^{2}+y^{2}=25$ with midpoint $(h, k)$ is given by $T=S_1$,which is $xh+yk=h^{2}+k^{2}$.
This can be rewritten as $y = -\frac{h}{k}x + \frac{h^{2}+k^{2}}{k}$.
This line is tangent to the hyperbola $\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$.
The condition for the line $y=mx+c$ to be a tangent to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ is $c^{2}=a^{2}m^{2}-b^{2}$.
Here,$m=-\frac{h}{k}$ and $c=\frac{h^{2}+k^{2}}{k}$,$a^{2}=9$,$b^{2}=16$.
Substituting these values into the condition: $\left(\frac{h^{2}+k^{2}}{k}\right)^{2} = 9\left(-\frac{h}{k}\right)^{2} - 16$.
$\frac{(h^{2}+k^{2})^{2}}{k^{2}} = \frac{9h^{2}}{k^{2}} - 16$.
$(h^{2}+k^{2})^{2} = 9h^{2} - 16k^{2}$.
Replacing $(h, k)$ with $(x, y)$,the locus is $(x^{2}+y^{2})^{2} = 9x^{2}-16y^{2}$,or $(x^{2}+y^{2})^{2}-9x^{2}+16y^{2}=0$.
Solution diagram
254
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola: $\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$.

Solution

(N/A) Comparing the given equation $\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$ with the standard equation $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,we get:
$a^{2} = 9 \implies a = 3$
$b^{2} = 16 \implies b = 4$
For a hyperbola,$c = \sqrt{a^{2} + b^{2}} = \sqrt{9 + 16} = \sqrt{25} = 5$.
$1$. The coordinates of the foci are $(\pm c, 0) = (\pm 5, 0)$.
$2$. The coordinates of the vertices are $(\pm a, 0) = (\pm 3, 0)$.
$3$. The eccentricity $e = \frac{c}{a} = \frac{5}{3}$.
$4$. The length of the latus rectum is $\frac{2b^{2}}{a} = \frac{2 \times 16}{3} = \frac{32}{3}$.
255
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola: $y^{2}-16x^{2}=16$.

Solution

(N/A) Dividing the equation by $16$ on both sides,we get $\frac{y^{2}}{16}-\frac{x^{2}}{1}=1$.
Comparing this with the standard equation $\frac{y^{2}}{a^{2}}-\frac{x^{2}}{b^{2}}=1$,we find $a^{2}=16$ and $b^{2}=1$,so $a=4$ and $b=1$.
Calculating $c$: $c = \sqrt{a^{2}+b^{2}} = \sqrt{16+1} = \sqrt{17}$.
The coordinates of the foci are $(0, \pm c) = (0, \pm \sqrt{17})$.
The coordinates of the vertices are $(0, \pm a) = (0, \pm 4)$.
The eccentricity $e = \frac{c}{a} = \frac{\sqrt{17}}{4}$.
The length of the latus rectum is $\frac{2b^{2}}{a} = \frac{2(1)}{4} = \frac{1}{2}$.
256
MediumMCQ
Find the equation of the hyperbola with foci $(0, \pm 3)$ and vertices $(0, \pm \frac{\sqrt{11}}{2})$.
A
$100 y^{2} - 44 x^{2} = 275$
B
$44 x^{2} - 100 y^{2} = 275$
C
$25 y^{2} - 11 x^{2} = 275$
D
$11 y^{2} - 25 x^{2} = 275$

Solution

(A) Since the foci are on the $y$-axis,the equation of the hyperbola is of the form $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$.
Given the vertices are $(0, \pm \frac{\sqrt{11}}{2})$,we have $a = \frac{\sqrt{11}}{2}$,so $a^{2} = \frac{11}{4}$.
Given the foci are $(0, \pm 3)$,we have $c = 3$,so $c^{2} = 9$.
For a hyperbola,$b^{2} = c^{2} - a^{2} = 9 - \frac{11}{4} = \frac{36 - 11}{4} = \frac{25}{4}$.
Substituting $a^{2}$ and $b^{2}$ into the standard equation:
$\frac{y^{2}}{11/4} - \frac{x^{2}}{25/4} = 1$
$\frac{4y^{2}}{11} - \frac{4x^{2}}{25} = 1$
Multiplying by $275$ (the $LCM$ of $11$ and $25$):
$100y^{2} - 44x^{2} = 275$.
257
MediumMCQ
Find the equation of the hyperbola whose foci are $(0, \pm 12)$ and the length of the latus rectum is $36$.
A
$3y^{2} - x^{2} = 108$
B
$x^{2} - 3y^{2} = 108$
C
$3x^{2} - y^{2} = 108$
D
$y^{2} - 3x^{2} = 108$

Solution

(A) Since the foci are $(0, \pm 12)$,the hyperbola is vertical and $c = 12$.
The length of the latus rectum is given by $\frac{2b^{2}}{a} = 36$,which implies $b^{2} = 18a$.
For a hyperbola,$c^{2} = a^{2} + b^{2}$. Substituting the values,we get $144 = a^{2} + 18a$.
Rearranging gives the quadratic equation $a^{2} + 18a - 144 = 0$.
Factoring the quadratic,we get $(a + 24)(a - 6) = 0$,so $a = -24$ or $a = 6$.
Since $a$ must be positive,we take $a = 6$. Then $b^{2} = 18(6) = 108$.
The equation of the hyperbola is $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$,which is $\frac{y^{2}}{36} - \frac{x^{2}}{108} = 1$.
Multiplying by $108$,we get $3y^{2} - x^{2} = 108$.
258
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $\frac{x^{2}}{16}-\frac{y^{2}}{9}=1$.

Solution

(N/A) The given equation is $\frac{x^{2}}{16}-\frac{y^{2}}{9}=1$,which can be written as $\frac{x^{2}}{4^{2}}-\frac{y^{2}}{3^{2}}=1$.
Comparing this with the standard equation $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,we get $a=4$ and $b=3$.
We know that $c^{2}=a^{2}+b^{2}$.
Substituting the values,$c^{2}=4^{2}+3^{2}=16+9=25$,so $c=5$.
$1$. The coordinates of the foci are $(\pm c, 0) = (\pm 5, 0)$.
$2$. The coordinates of the vertices are $(\pm a, 0) = (\pm 4, 0)$.
$3$. The eccentricity $e = \frac{c}{a} = \frac{5}{4}$.
$4$. The length of the latus rectum $= \frac{2b^{2}}{a} = \frac{2 \times 9}{4} = \frac{9}{2} = 4.5$.
259
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $\frac{y^{2}}{9}-\frac{x^{2}}{27}=1$.

Solution

(N/A) The given equation is $\frac{y^{2}}{9}-\frac{x^{2}}{27}=1$,which can be written as $\frac{y^{2}}{3^{2}}-\frac{x^{2}}{(\sqrt{27})^{2}}=1$.
Comparing this with the standard equation of a vertical hyperbola $\frac{y^{2}}{a^{2}}-\frac{x^{2}}{b^{2}}=1$,we get $a=3$ and $b=\sqrt{27}$.
For a hyperbola,the relation between $a, b,$ and $c$ is $c^{2}=a^{2}+b^{2}$.
Substituting the values,$c^{2}=3^{2}+(\sqrt{27})^{2}=9+27=36$,so $c=6$.
The coordinates of the foci are $(0, \pm c) = (0, \pm 6)$.
The coordinates of the vertices are $(0, \pm a) = (0, \pm 3)$.
The eccentricity $e$ is given by $e=\frac{c}{a}=\frac{6}{3}=2$.
The length of the latus rectum is $\frac{2b^{2}}{a}=\frac{2 \times 27}{3}=18$.
260
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $9 y^{2}-4 x^{2}=36$.

Solution

(N/A) The given equation is $9 y^{2}-4 x^{2}=36$.
Dividing both sides by $36$,we get:
$\frac{9 y^{2}}{36} - \frac{4 x^{2}}{36} = 1$
$\frac{y^{2}}{4} - \frac{x^{2}}{9} = 1$
This is the standard form of a hyperbola $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$,where $a^{2} = 4$ and $b^{2} = 9$.
Thus,$a = 2$ and $b = 3$.
For a hyperbola,$c^{2} = a^{2} + b^{2} = 4 + 9 = 13$,so $c = \sqrt{13}$.
$1$. The coordinates of the foci are $(0, \pm c) = (0, \pm \sqrt{13})$.
$2$. The coordinates of the vertices are $(0, \pm a) = (0, \pm 2)$.
$3$. The eccentricity $e = \frac{c}{a} = \frac{\sqrt{13}}{2}$.
$4$. The length of the latus rectum $= \frac{2 b^{2}}{a} = \frac{2 \times 9}{2} = 9$.
261
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $16 x^{2}-9 y^{2}=576$.

Solution

(N/A) The given equation is $16 x^{2}-9 y^{2}=576$.
It can be written as:
$\frac{x^{2}}{36}-\frac{y^{2}}{64}=1$
$\Rightarrow \frac{x^{2}}{6^{2}}-\frac{y^{2}}{8^{2}}=1$ $(1)$
On comparing equation $(1)$ with the standard equation of the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,we obtain $a=6$ and $b=8$.
We know that $c^{2}=a^{2}+b^{2}$.
$\therefore c^{2}=36+64=100$
$\Rightarrow c=10$.
Therefore:
$1$. The coordinates of the foci are $(\pm 10, 0)$.
$2$. The coordinates of the vertices are $(\pm 6, 0)$.
$3$. Eccentricity,$e = \frac{c}{a} = \frac{10}{6} = \frac{5}{3}$.
$4$. Length of the latus rectum $= \frac{2b^{2}}{a} = \frac{2 \times 64}{6} = \frac{64}{3}$.
262
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $5y^{2} - 9x^{2} = 36$.

Solution

The given equation is $5y^{2} - 9x^{2} = 36$.
Dividing by $36$,we get $\frac{y^{2}}{(36/5)} - \frac{x^{2}}{4} = 1$.
Comparing this with the standard equation $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$,we have $a^{2} = \frac{36}{5}$ and $b^{2} = 4$.
Thus,$a = \frac{6}{\sqrt{5}}$ and $b = 2$.
For a hyperbola,$c^{2} = a^{2} + b^{2} = \frac{36}{5} + 4 = \frac{56}{5}$.
So,$c = \sqrt{\frac{56}{5}} = \frac{2\sqrt{14}}{\sqrt{5}}$.
Coordinates of the foci are $(0, \pm c) = (0, \pm \frac{2\sqrt{14}}{\sqrt{5}})$.
Coordinates of the vertices are $(0, \pm a) = (0, \pm \frac{6}{\sqrt{5}})$.
Eccentricity $e = \frac{c}{a} = \frac{2\sqrt{14}/\sqrt{5}}{6/\sqrt{5}} = \frac{\sqrt{14}}{3}$.
Length of the latus rectum $= \frac{2b^{2}}{a} = \frac{2 \times 4}{6/\sqrt{5}} = \frac{4\sqrt{5}}{3}$.
263
Medium
Find the coordinates of the foci and the vertices,the eccentricity,and the length of the latus rectum of the hyperbola $49 y^{2}-16 x^{2}=784$.

Solution

(N/A) The given equation is $49 y^{2}-16 x^{2}=784$.
Dividing both sides by $784$,we get:
$\frac{49 y^{2}}{784} - \frac{16 x^{2}}{784} = 1$
$\frac{y^{2}}{16} - \frac{x^{2}}{49} = 1$
$\frac{y^{2}}{4^{2}} - \frac{x^{2}}{7^{2}} = 1$ $(1)$
Comparing equation $(1)$ with the standard equation of the hyperbola $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$,we get $a = 4$ and $b = 7$.
We know that $c^{2} = a^{2} + b^{2}$.
$c^{2} = 16 + 49 = 65$
$c = \sqrt{65}$
Coordinates of the foci are $(0, \pm \sqrt{65})$.
Coordinates of the vertices are $(0, \pm 4)$.
Eccentricity $e = \frac{c}{a} = \frac{\sqrt{65}}{4}$.
Length of the latus rectum $= \frac{2 b^{2}}{a} = \frac{2 \times 49}{4} = \frac{49}{2} = 24.5$.
264
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Vertices $(\pm 2, 0)$,foci $(\pm 3, 0)$.
A
$\frac{x^{2}}{4} - \frac{y^{2}}{5} = 1$
B
$\frac{x^{2}}{5} - \frac{y^{2}}{4} = 1$
C
$\frac{x^{2}}{4} + \frac{y^{2}}{5} = 1$
D
$\frac{x^{2}}{9} - \frac{y^{2}}{4} = 1$

Solution

(A) The vertices are $(\pm 2, 0)$,which lie on the $x$-axis.
Thus,the equation of the hyperbola is of the form $\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1$.
Given vertices $(\pm a, 0) = (\pm 2, 0)$,we have $a = 2$,so $a^{2} = 4$.
Given foci $(\pm c, 0) = (\pm 3, 0)$,we have $c = 3$,so $c^{2} = 9$.
For a hyperbola,we know the relation $c^{2} = a^{2} + b^{2}$.
Substituting the values,$9 = 4 + b^{2}$,which gives $b^{2} = 5$.
Therefore,the equation of the hyperbola is $\frac{x^{2}}{4} - \frac{y^{2}}{5} = 1$.
265
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Vertices $(0, \pm 5)$,foci $(0, \pm 8)$.
A
$\frac{y^{2}}{25} - \frac{x^{2}}{39} = 1$
B
$\frac{y^{2}}{39} - \frac{x^{2}}{25} = 1$
C
$\frac{x^{2}}{25} - \frac{y^{2}}{39} = 1$
D
$\frac{x^{2}}{39} - \frac{y^{2}}{25} = 1$

Solution

(A) The vertices are $(0, \pm 5)$,which lie on the $y$-axis.
Thus,the equation of the hyperbola is of the form $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$.
Given the vertices $(0, \pm a) = (0, \pm 5)$,we have $a = 5$,so $a^{2} = 25$.
Given the foci $(0, \pm c) = (0, \pm 8)$,we have $c = 8$,so $c^{2} = 64$.
For a hyperbola,we know the relation $c^{2} = a^{2} + b^{2}$.
Substituting the values,$64 = 25 + b^{2}$,which gives $b^{2} = 64 - 25 = 39$.
Substituting $a^{2}$ and $b^{2}$ into the standard form,the equation is $\frac{y^{2}}{25} - \frac{x^{2}}{39} = 1$.
266
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Vertices $(0, \pm 3)$,foci $(0, \pm 5)$.
A
$\frac{y^{2}}{9} - \frac{x^{2}}{16} = 1$
B
$\frac{x^{2}}{9} - \frac{y^{2}}{16} = 1$
C
$\frac{y^{2}}{16} - \frac{x^{2}}{9} = 1$
D
$\frac{x^{2}}{16} - \frac{y^{2}}{9} = 1$

Solution

(A) The vertices are $(0, \pm 3)$,which implies the hyperbola is along the $y$-axis.
The standard equation is $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$.
Given vertices $(0, \pm a) = (0, \pm 3)$,we have $a = 3$,so $a^{2} = 9$.
Given foci $(0, \pm c) = (0, \pm 5)$,we have $c = 5$,so $c^{2} = 25$.
For a hyperbola,$c^{2} = a^{2} + b^{2}$.
Substituting the values: $25 = 9 + b^{2}$.
$b^{2} = 25 - 9 = 16$.
Thus,the equation is $\frac{y^{2}}{9} - \frac{x^{2}}{16} = 1$.
267
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Foci $(\pm 5, 0)$,the transverse axis is of length $8$.
A
$\frac{x^{2}}{16} - \frac{y^{2}}{9} = 1$
B
$\frac{x^{2}}{9} - \frac{y^{2}}{16} = 1$
C
$\frac{x^{2}}{25} - \frac{y^{2}}{9} = 1$
D
$\frac{x^{2}}{16} + \frac{y^{2}}{9} = 1$

Solution

(A) The foci are $(\pm 5, 0)$,which lie on the $x$-axis. The standard form of the hyperbola is $\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1$.
Given the foci $(\pm c, 0) = (\pm 5, 0)$,we have $c = 5$.
The length of the transverse axis is $2a = 8$,which gives $a = 4$.
Using the relation $c^{2} = a^{2} + b^{2}$,we get $5^{2} = 4^{2} + b^{2}$.
$25 = 16 + b^{2} \Rightarrow b^{2} = 25 - 16 = 9$.
Substituting $a^{2} = 16$ and $b^{2} = 9$ into the standard equation,we get $\frac{x^{2}}{16} - \frac{y^{2}}{9} = 1$.
268
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Foci $(0, \pm 13)$,the conjugate axis is of length $24$.
A
$\frac{y^{2}}{25} - \frac{x^{2}}{144} = 1$
B
$\frac{x^{2}}{25} - \frac{y^{2}}{144} = 1$
C
$\frac{y^{2}}{144} - \frac{x^{2}}{25} = 1$
D
$\frac{x^{2}}{144} - \frac{y^{2}}{25} = 1$

Solution

(A) Given the foci are $(0, \pm 13)$,the hyperbola is vertical and centered at the origin. The standard form is $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$.
Since the foci are $(0, \pm c)$,we have $c = 13$.
The length of the conjugate axis is $2b = 24$,which implies $b = 12$.
Using the relation $c^{2} = a^{2} + b^{2}$,we get $13^{2} = a^{2} + 12^{2}$.
$169 = a^{2} + 144 \Rightarrow a^{2} = 169 - 144 = 25$.
Substituting $a^{2} = 25$ and $b^{2} = 144$ into the standard equation,we get $\frac{y^{2}}{25} - \frac{x^{2}}{144} = 1$.
269
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Foci $(\pm 3 \sqrt{5}, 0)$,the latus rectum is of length $8$.
A
$\frac{x^{2}}{25} - \frac{y^{2}}{20} = 1$
B
$\frac{x^{2}}{20} - \frac{y^{2}}{25} = 1$
C
$\frac{x^{2}}{16} - \frac{y^{2}}{9} = 1$
D
$\frac{x^{2}}{9} - \frac{y^{2}}{16} = 1$

Solution

(A) Given,the foci are $(\pm 3 \sqrt{5}, 0)$. Since the foci lie on the $x$-axis,the equation of the hyperbola is of the form $\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1$.
Here,$c = 3 \sqrt{5}$,so $c^{2} = (3 \sqrt{5})^{2} = 9 \times 5 = 45$.
The length of the latus rectum is given as $8$,so $\frac{2b^{2}}{a} = 8$,which implies $b^{2} = 4a$.
We know the relation $c^{2} = a^{2} + b^{2}$. Substituting the values,we get $a^{2} + 4a = 45$,or $a^{2} + 4a - 45 = 0$.
Factoring the quadratic equation: $(a + 9)(a - 5) = 0$. Since $a > 0$,we have $a = 5$.
Then $b^{2} = 4a = 4 \times 5 = 20$.
Substituting $a^{2} = 25$ and $b^{2} = 20$ into the standard equation,we get $\frac{x^{2}}{25} - \frac{y^{2}}{20} = 1$.
270
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Foci $(\pm 4, 0)$,the latus rectum is of length $12$.
A
$\frac{x^{2}}{4} - \frac{y^{2}}{12} = 1$
B
$\frac{x^{2}}{8} - \frac{y^{2}}{4} = 1$
C
$\frac{x^{2}}{12} - \frac{y^{2}}{4} = 1$
D
$\frac{x^{2}}{16} - \frac{y^{2}}{12} = 1$

Solution

(A) The foci are $(\pm 4, 0)$,which lie on the $x$-axis. The standard equation of the hyperbola is $\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1$.
Given the foci are $(\pm c, 0)$,we have $c = 4$,so $c^{2} = 16$.
The length of the latus rectum is given by $\frac{2b^{2}}{a} = 12$,which implies $b^{2} = 6a$.
Using the relation $c^{2} = a^{2} + b^{2}$,we substitute $c^{2} = 16$ and $b^{2} = 6a$:
$a^{2} + 6a = 16$
$a^{2} + 6a - 16 = 0$
$(a + 8)(a - 2) = 0$
Since $a > 0$,we have $a = 2$.
Then $b^{2} = 6(2) = 12$.
Substituting $a^{2} = 4$ and $b^{2} = 12$ into the standard equation,we get $\frac{x^{2}}{4} - \frac{y^{2}}{12} = 1$.
271
MediumMCQ
Find the equation of the hyperbola satisfying the given conditions: Vertices $(\pm 7, 0)$,$e = \frac{4}{3}$.
A
$\frac{x^2}{49} - \frac{9y^2}{343} = 1$
B
$\frac{x^2}{49} - \frac{y^2}{343} = 1$
C
$\frac{x^2}{343} - \frac{y^2}{49} = 1$
D
$\frac{x^2}{49} + \frac{y^2}{343} = 1$

Solution

(A) The vertices are $(\pm 7, 0)$,which lie on the $x$-axis. Thus,the equation is of the form $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$.
Given $a = 7$ and eccentricity $e = \frac{c}{a} = \frac{4}{3}$.
Substituting $a = 7$,we get $\frac{c}{7} = \frac{4}{3}$,so $c = \frac{28}{3}$.
Using the relation $c^2 = a^2 + b^2$,we have $b^2 = c^2 - a^2 = (\frac{28}{3})^2 - 7^2 = \frac{784}{9} - 49 = \frac{784 - 441}{9} = \frac{343}{9}$.
Substituting $a^2 = 49$ and $b^2 = \frac{343}{9}$ into the standard equation,we get $\frac{x^2}{49} - \frac{y^2}{343/9} = 1$,which simplifies to $\frac{x^2}{49} - \frac{9y^2}{343} = 1$.
272
DifficultMCQ
Find the equation of the hyperbola satisfying the given conditions: Foci $(0, \pm \sqrt{10})$,passing through $(2, 3)$.
A
$\frac{y^{2}}{5} - \frac{x^{2}}{5} = 1$
B
$\frac{y^{2}}{9} - \frac{x^{2}}{1} = 1$
C
$\frac{y^{2}}{4} - \frac{x^{2}}{6} = 1$
D
$\frac{y^{2}}{2} - \frac{x^{2}}{8} = 1$

Solution

(A) The foci are $(0, \pm \sqrt{10})$,which lie on the $y$-axis.
Thus,the equation is of the form $\frac{y^{2}}{a^{2}} - \frac{x^{2}}{b^{2}} = 1$.
Here,$c = \sqrt{10}$,so $c^{2} = 10$.
We know $c^{2} = a^{2} + b^{2}$,so $a^{2} + b^{2} = 10$,which means $b^{2} = 10 - a^{2}$.
The hyperbola passes through $(2, 3)$,so $\frac{3^{2}}{a^{2}} - \frac{2^{2}}{b^{2}} = 1$,or $\frac{9}{a^{2}} - \frac{4}{b^{2}} = 1$.
Substituting $b^{2} = 10 - a^{2}$: $\frac{9}{a^{2}} - \frac{4}{10 - a^{2}} = 1$.
$9(10 - a^{2}) - 4a^{2} = a^{2}(10 - a^{2})$.
$90 - 9a^{2} - 4a^{2} = 10a^{2} - a^{4}$.
$a^{4} - 23a^{2} + 90 = 0$.
$(a^{2} - 18)(a^{2} - 5) = 0$.
Since $c^{2} > a^{2}$,$10 > a^{2}$,so $a^{2} = 5$.
Then $b^{2} = 10 - 5 = 5$.
The equation is $\frac{y^{2}}{5} - \frac{x^{2}}{5} = 1$.
273
DifficultMCQ
$A$ line parallel to the straight line $2x - y = 0$ is tangent to the hyperbola $\frac{x^{2}}{4} - \frac{y^{2}}{2} = 1$ at the point $(x_{1}, y_{1})$. Then $x_{1}^{2} + 5y_{1}^{2}$ is equal to:
A
$5$
B
$6$
C
$8$
D
$10$

Solution

(B) The slope of the line $2x - y = 0$ is $2$. Since the tangent is parallel to this line,its slope is $m = 2$.
The equation of the tangent to the hyperbola $\frac{x^{2}}{4} - \frac{y^{2}}{2} = 1$ at the point $(x_{1}, y_{1})$ is given by $\frac{xx_{1}}{4} - \frac{yy_{1}}{2} = 1$.
The slope of this tangent is $\frac{x_{1}/4}{y_{1}/2} = \frac{x_{1}}{2y_{1}}$.
Equating the slopes: $\frac{x_{1}}{2y_{1}} = 2 \Rightarrow x_{1} = 4y_{1} \quad (1)$.
Since $(x_{1}, y_{1})$ lies on the hyperbola,we have $\frac{x_{1}^{2}}{4} - \frac{y_{1}^{2}}{2} = 1 \quad (2)$.
Substituting $(1)$ into $(2)$: $\frac{(4y_{1})^{2}}{4} - \frac{y_{1}^{2}}{2} = 1 \Rightarrow 4y_{1}^{2} - \frac{y_{1}^{2}}{2} = 1$.
$\frac{7y_{1}^{2}}{2} = 1 \Rightarrow y_{1}^{2} = \frac{2}{7}$.
Now,calculate $x_{1}^{2} + 5y_{1}^{2} = (4y_{1})^{2} + 5y_{1}^{2} = 16y_{1}^{2} + 5y_{1}^{2} = 21y_{1}^{2}$.
Substituting $y_{1}^{2} = \frac{2}{7}$: $21 \times \frac{2}{7} = 3 \times 2 = 6$.
274
DifficultMCQ
$A$ hyperbola having the transverse axis of length $\sqrt{2}$ has the same foci as that of the ellipse $3x^{2} + 4y^{2} = 12$. Then this hyperbola does not pass through which of the following points?
A
$\left(1, -\frac{1}{\sqrt{2}}\right)$
B
$\left(\sqrt{\frac{3}{2}}, \frac{1}{\sqrt{2}}\right)$
C
$\left(\frac{1}{\sqrt{2}}, 0\right)$
D
$\left(-\sqrt{\frac{3}{2}}, 1\right)$

Solution

(B) The given ellipse is $3x^{2} + 4y^{2} = 12$,which can be written as $\frac{x^{2}}{4} + \frac{y^{2}}{3} = 1$.
Here,$a^{2} = 4$ and $b^{2} = 3$. The eccentricity $e = \sqrt{1 - \frac{b^{2}}{a^{2}}} = \sqrt{1 - \frac{3}{4}} = \frac{1}{2}$.
The foci are $(\pm ae, 0) = (\pm 2 \times \frac{1}{2}, 0) = (\pm 1, 0)$.
For the hyperbola,the length of the transverse axis is $2a_{h} = \sqrt{2}$,so $a_{h} = \frac{1}{\sqrt{2}}$ and $a_{h}^{2} = \frac{1}{2}$.
Let the hyperbola be $\frac{x^{2}}{a_{h}^{2}} - \frac{y^{2}}{b_{h}^{2}} = 1$,i.e.,$\frac{x^{2}}{1/2} - \frac{y^{2}}{b_{h}^{2}} = 1$.
The foci of the hyperbola are $(\pm a_{h}e_{h}, 0)$,where $e_{h} = \sqrt{1 + \frac{b_{h}^{2}}{a_{h}^{2}}} = \sqrt{1 + 2b_{h}^{2}}$.
Thus,the foci are $(\pm \sqrt{a_{h}^{2} + b_{h}^{2}}, 0) = (\pm \sqrt{\frac{1}{2} + b_{h}^{2}}, 0)$.
Since the foci are the same,$\sqrt{\frac{1}{2} + b_{h}^{2}} = 1$,which implies $\frac{1}{2} + b_{h}^{2} = 1$,so $b_{h}^{2} = \frac{1}{2}$.
The equation of the hyperbola is $\frac{x^{2}}{1/2} - \frac{y^{2}}{1/2} = 1$,or $x^{2} - y^{2} = \frac{1}{2}$.
Checking the points:
For option $B$: $(\sqrt{3/2})^{2} - (1/\sqrt{2})^{2} = \frac{3}{2} - \frac{1}{2} = 1 \neq \frac{1}{2}$.
Thus,the point $\left(\sqrt{\frac{3}{2}}, \frac{1}{\sqrt{2}}\right)$ does not lie on the hyperbola.
275
DifficultMCQ
Let $P(3,3)$ be a point on the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$. If the normal to it at $P$ intersects the $x$-axis at $(9,0)$ and $e$ is its eccentricity,then the ordered pair $(a^{2}, e^{2})$ is equal to
A
$(\frac{9}{2}, 3)$
B
$(\frac{9}{2}, 2)$
C
$(\frac{3}{2}, 2)$
D
$(9, 3)$

Solution

(A) Since the point $(3,3)$ lies on the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,we have $\frac{9}{a^{2}}-\frac{9}{b^{2}}=1$ $(i)$.
The equation of the normal at $(x_{1}, y_{1})$ to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ is $\frac{a^{2}x}{x_{1}} + \frac{b^{2}y}{y_{1}} = a^{2} + b^{2}$.
Substituting $(x_{1}, y_{1}) = (3,3)$,the normal is $\frac{a^{2}x}{3} + \frac{b^{2}y}{3} = a^{2} + b^{2}$.
This normal passes through $(9,0)$,so $\frac{a^{2}(9)}{3} + 0 = a^{2} + b^{2}$ $\Rightarrow 3a^{2} = a^{2} + b^{2}$ $\Rightarrow b^{2} = 2a^{2}$ $(ii)$.
Substituting $(ii)$ into $(i)$: $\frac{9}{a^{2}} - \frac{9}{2a^{2}} = 1$ $\Rightarrow \frac{18-9}{2a^{2}} = 1$ $\Rightarrow 9 = 2a^{2}$ $\Rightarrow a^{2} = \frac{9}{2}$.
Then $b^{2} = 2(\frac{9}{2}) = 9$.
The eccentricity $e$ is given by $e^{2} = 1 + \frac{b^{2}}{a^{2}} = 1 + \frac{9}{9/2} = 1 + 2 = 3$.
Thus,the ordered pair $(a^{2}, e^{2})$ is $(\frac{9}{2}, 3)$.
276
DifficultMCQ
If the line $y=mx+c$ is a common tangent to the hyperbola $\frac{x^{2}}{100}-\frac{y^{2}}{64}=1$ and the circle $x^{2}+y^{2}=36,$ then which one of the following is true?
A
$5m=4$
B
$4c^{2}=369$
C
$c^{2}=369$
D
$8m+5=0$

Solution

(B) The condition for the line $y=mx+c$ to be a tangent to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ is $c^{2}=a^{2}m^{2}-b^{2}$.
For the given hyperbola $\frac{x^{2}}{100}-\frac{y^{2}}{64}=1$,we have $a^{2}=100$ and $b^{2}=64$,so $c^{2}=100m^{2}-64$.
The condition for the line $y=mx+c$ to be a tangent to the circle $x^{2}+y^{2}=r^{2}$ is $c^{2}=r^{2}(1+m^{2})$.
For the given circle $x^{2}+y^{2}=36$,we have $r^{2}=36$,so $c^{2}=36(1+m^{2})$.
Equating the two expressions for $c^{2}$:
$100m^{2}-64=36(1+m^{2})$
$100m^{2}-64=36+36m^{2}$
$64m^{2}=100$
$m^{2}=\frac{100}{64}=\frac{25}{16}$.
Now,substitute $m^{2}$ back into the circle's condition:
$c^{2}=36(1+\frac{25}{16})$
$c^{2}=36(\frac{16+25}{16})$
$c^{2}=36(\frac{41}{16})$
$c^{2}=\frac{9 \times 41}{4}$
$4c^{2}=369$.
277
DifficultMCQ
$A$ square $ABCD$ has all its vertices on the curve $x^{2}y^{2}=1$. The midpoints of its sides also lie on the same curve. Then,the square of the area of $ABCD$ is
A
$70$
B
$96$
C
$75$
D
$80$

Solution

(D) The curve is $x^{2}y^{2}=1$,which implies $xy=1$ or $xy=-1$. Let the vertices be $A(t_1, 1/t_1)$,$B(t_2, -1/t_2)$,$C(-t_1, -1/t_1)$,and $D(-t_2, 1/t_2)$.
Since $ABCD$ is a square,the midpoint of $AB$ must lie on the curve $xy=1$ or $xy=-1$. The midpoint of $AB$ is $M = (\frac{t_1+t_2}{2}, \frac{1/t_1 - 1/t_2}{2})$.
For $M$ to lie on $xy=1$,we have $(\frac{t_1+t_2}{2})(\frac{t_2-t_1}{2t_1t_2}) = 1$,which simplifies to $t_2^2 - t_1^2 = 4t_1t_2$.
Also,the slope of $AB$ must be $-1$ (since $ABCD$ is a square and the vertices are symmetric about the origin). The slope of $AB$ is $\frac{-1/t_2 - 1/t_1}{t_2 - t_1} = \frac{-(t_1+t_2)}{t_1t_2(t_2-t_1)} = -1$,which implies $t_1+t_2 = t_1t_2(t_2-t_1)$.
Solving these equations,we find $t_1t_2 = 1$ and $t_2^2 - t_1^2 = 4$. Thus $t_2^2 + t_1^2 = \sqrt{4^2 + 4(1)^2} = \sqrt{20} = 2\sqrt{5}$.
The side length squared is $AB^2 = (t_2-t_1)^2 + (-1/t_2 - 1/t_1)^2 = (t_2-t_1)^2 + (\frac{t_1+t_2}{t_1t_2})^2 = (t_2^2+t_1^2 - 2t_1t_2) + (t_1+t_2)^2 = (2\sqrt{5}-2) + (2\sqrt{5}+2) = 4\sqrt{5}$.
Area of $ABCD = AB^2 = 4\sqrt{5}$.
Therefore,the square of the area is $(4\sqrt{5})^2 = 16 \times 5 = 80$.
Solution diagram
278
DifficultMCQ
Consider a hyperbola $H : x^{2}-2y^{2}=4$. Let the tangent at a point $P(4, \sqrt{6})$ meet the $x$-axis at $Q$ and the latus rectum at $R(x_{1}, y_{1})$,where $x_{1}>0$. If $F$ is a focus of $H$ which is nearer to the point $P$,then the area of $\Delta QFR$ is equal to ....... .
A
$4\sqrt{6}$
B
$\sqrt{6}-1$
C
$\frac{7}{\sqrt{6}}-2$
D
$4\sqrt{6}-1$

Solution

(C) The given hyperbola is $x^{2}-2y^{2}=4$,which can be written as $\frac{x^{2}}{4}-\frac{y^{2}}{2}=1$.
Here,$a^{2}=4$ and $b^{2}=2$.
The eccentricity $e$ is given by $e=\sqrt{1+\frac{b^{2}}{a^{2}}}=\sqrt{1+\frac{2}{4}}=\sqrt{\frac{3}{2}}$.
The focus $F$ is $(ae, 0) = (2 \cdot \sqrt{\frac{3}{2}}, 0) = (\sqrt{6}, 0)$.
The equation of the tangent at $P(4, \sqrt{6})$ to the hyperbola $\frac{x^{2}}{4}-\frac{y^{2}}{2}=1$ is $\frac{x x_{1}}{4}-\frac{y y_{1}}{2}=1$.
Substituting $(x_{1}, y_{1}) = (4, \sqrt{6})$,we get $\frac{4x}{4}-\frac{\sqrt{6}y}{2}=1$,which simplifies to $x - \frac{\sqrt{6}}{2}y = 1$,or $2x - y\sqrt{6} = 2$.
To find $Q$,set $y=0$ in the tangent equation: $2x=2 \Rightarrow x=1$. So,$Q(1, 0)$.
To find $R$,substitute $x=\sqrt{6}$ (the latus rectum) into the tangent equation: $2(\sqrt{6}) - y\sqrt{6} = 2 \Rightarrow y\sqrt{6} = 2\sqrt{6}-2 \Rightarrow y = 2 - \frac{2}{\sqrt{6}}$.
Thus,$R(\sqrt{6}, 2 - \frac{2}{\sqrt{6}})$.
The area of $\Delta QFR$ with vertices $Q(1, 0)$,$F(\sqrt{6}, 0)$,and $R(\sqrt{6}, 2 - \frac{2}{\sqrt{6}})$ is $\frac{1}{2} \times \text{base} \times \text{height}$.
Base $QF = |\sqrt{6}-1|$.
Height $FR = |2 - \frac{2}{\sqrt{6}}|$.
Area $= \frac{1}{2} \times (\sqrt{6}-1) \times 2(1 - \frac{1}{\sqrt{6}}) = (\sqrt{6}-1) \times \frac{\sqrt{6}-1}{\sqrt{6}} = \frac{(\sqrt{6}-1)^{2}}{\sqrt{6}} = \frac{6+1-2\sqrt{6}}{\sqrt{6}} = \frac{7}{\sqrt{6}}-2$.
Solution diagram
279
DifficultMCQ
The locus of the point of intersection of the lines $(\sqrt{3})kx + ky - 4\sqrt{3} = 0$ and $\sqrt{3}x - y - 4\sqrt{3}k = 0$ is a conic,whose eccentricity is .............
A
$0$
B
$2$
C
$4$
D
$8$

Solution

(B) Given lines are $(\sqrt{3})kx + ky = 4\sqrt{3}$ and $\sqrt{3}x - y = 4\sqrt{3}k$.
From the first equation,$k = \frac{4\sqrt{3}}{\sqrt{3}x + y}$.
From the second equation,$k = \frac{\sqrt{3}x - y}{4\sqrt{3}}$.
Equating the two expressions for $k$:
$\frac{4\sqrt{3}}{\sqrt{3}x + y} = \frac{\sqrt{3}x - y}{4\sqrt{3}}$
$(\sqrt{3}x - y)(\sqrt{3}x + y) = 16(3) = 48$
$3x^2 - y^2 = 48$
Dividing by $48$,we get $\frac{x^2}{16} - \frac{y^2}{48} = 1$.
This is the equation of a hyperbola with $a^2 = 16$ and $b^2 = 48$.
The eccentricity $e$ is given by $b^2 = a^2(e^2 - 1)$.
$48 = 16(e^2 - 1)$
$3 = e^2 - 1$
$e^2 = 4$
$e = 2$.
280
MediumMCQ
$A$ hyperbola passes through the foci of the ellipse $\frac{x^{2}}{25}+\frac{y^{2}}{16}=1$ and its transverse and conjugate axes coincide with the major and minor axes of the ellipse,respectively. If the product of their eccentricities is $1$,then the equation of the hyperbola is ...... .
A
$\frac{x^{2}}{9}-\frac{y^{2}}{25}=1$
B
$\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$
C
$x^{2}-y^{2}=9$
D
$\frac{x^{2}}{9}-\frac{y^{2}}{4}=1$

Solution

(B) For the ellipse $\frac{x^{2}}{25}+\frac{y^{2}}{16}=1$,we have $a^{2}=25$ and $b^{2}=16$.
The eccentricity $e_{1} = \sqrt{1-\frac{b^{2}}{a^{2}}} = \sqrt{1-\frac{16}{25}} = \sqrt{\frac{9}{25}} = \frac{3}{5}$.
The foci of the ellipse are $(\pm ae_{1}, 0) = (\pm 5 \times \frac{3}{5}, 0) = (\pm 3, 0)$.
Given that the product of the eccentricities of the ellipse and the hyperbola is $1$,we have $e_{1} \times e_{2} = 1$ $\Rightarrow \frac{3}{5} \times e_{2} = 1$ $\Rightarrow e_{2} = \frac{5}{3}$.
Let the equation of the hyperbola be $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$.
Since the hyperbola passes through the foci of the ellipse $(\pm 3, 0)$,we have $\frac{3^{2}}{a^{2}} = 1 \Rightarrow a^{2} = 9$.
For a hyperbola,$b^{2} = a^{2}(e_{2}^{2}-1) = 9((\frac{5}{3})^{2}-1) = 9(\frac{25}{9}-1) = 9(\frac{16}{9}) = 16$.
Thus,the equation of the hyperbola is $\frac{x^{2}}{9}-\frac{y^{2}}{16}=1$.
Solution diagram
281
DifficultMCQ
The point $P(-2 \sqrt{6}, \sqrt{3})$ lies on the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ having eccentricity $e = \frac{\sqrt{5}}{2}$. If the tangent and normal at $P$ to the hyperbola intersect its conjugate axis at the points $Q$ and $R$ respectively,then $QR$ is equal to :
A
$4 \sqrt{3}$
B
$6$
C
$6 \sqrt{3}$
D
$2 \sqrt{3}$

Solution

(C) Given the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ and eccentricity $e = \frac{\sqrt{5}}{2}$.
We know $b^{2} = a^{2}(e^{2}-1) = a^{2}(\frac{5}{4}-1) = \frac{a^{2}}{4}$,so $a^{2} = 4b^{2}$.
Since $P(-2 \sqrt{6}, \sqrt{3})$ lies on the hyperbola,$\frac{(-2 \sqrt{6})^{2}}{4b^{2}} - \frac{(\sqrt{3})^{2}}{b^{2}} = 1$.
$\frac{24}{4b^{2}} - \frac{3}{b^{2}} = 1 \Rightarrow \frac{6}{b^{2}} - \frac{3}{b^{2}} = 1 \Rightarrow \frac{3}{b^{2}} = 1 \Rightarrow b^{2} = 3$,so $b = \sqrt{3}$ and $a^{2} = 12$,so $a = 2 \sqrt{3}$.
The hyperbola is $\frac{x^{2}}{12} - \frac{y^{2}}{3} = 1$.
The tangent at $P(x_{1}, y_{1})$ is $\frac{xx_{1}}{a^{2}} - \frac{yy_{1}}{b^{2}} = 1$.
$\frac{x(-2 \sqrt{6})}{12} - \frac{y(\sqrt{3})}{3} = 1 \Rightarrow -\frac{x \sqrt{6}}{6} - \frac{y}{\sqrt{3}} = 1$.
For the conjugate axis,set $x = 0$: $-\frac{y}{\sqrt{3}} = 1 \Rightarrow y = -\sqrt{3}$. Thus $Q = (0, -\sqrt{3})$.
The slope of the tangent is $m = \frac{b^{2}x_{1}}{a^{2}y_{1}} = \frac{3(-2 \sqrt{6})}{12(\sqrt{3})} = \frac{-6 \sqrt{6}}{12 \sqrt{3}} = -\frac{\sqrt{2}}{2} = -\frac{1}{\sqrt{2}}$.
The slope of the normal is $m' = -\frac{1}{m} = \sqrt{2}$.
The equation of the normal at $P$ is $y - \sqrt{3} = \sqrt{2}(x + 2 \sqrt{6})$.
For the conjugate axis,set $x = 0$: $y - \sqrt{3} = \sqrt{2}(2 \sqrt{6}) = 2 \sqrt{12} = 4 \sqrt{3}$.
$y = 4 \sqrt{3} + \sqrt{3} = 5 \sqrt{3}$. Thus $R = (0, 5 \sqrt{3})$.
The distance $QR = |5 \sqrt{3} - (-\sqrt{3})| = |6 \sqrt{3}| = 6 \sqrt{3}$.
Solution diagram
282
DifficultMCQ
Let $A(\sec \theta, 2 \tan \theta)$ and $B(\sec \phi, 2 \tan \phi)$,where $\theta+\phi=\pi/2$,be two points on the hyperbola $2x^2-y^2=2$. If $(\alpha, \beta)$ is the point of intersection of the normals to the hyperbola at $A$ and $B$,then $(2\beta)^2$ is equal to ..... .
A
$6$
B
$12$
C
$24$
D
None of these

Solution

(D) The equation of the hyperbola is $2x^2 - y^2 = 2$,which can be written as $\frac{x^2}{1} - \frac{y^2}{2} = 1$. Here $a^2 = 1$ and $b^2 = 2$.
The normal at point $(x_1, y_1)$ on the hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ is $\frac{a^2x}{x_1} + \frac{b^2y}{y_1} = a^2 + b^2$.
For point $A(\sec \theta, 2 \tan \theta)$,the normal is $\frac{1 \cdot x}{\sec \theta} + \frac{2 \cdot y}{2 \tan \theta} = 1 + 2 = 3$,which simplifies to $x \cos \theta + y \cot \theta = 3$.
For point $B(\sec \phi, 2 \tan \phi)$,the normal is $x \cos \phi + y \cot \phi = 3$.
Given $\phi = \frac{\pi}{2} - \theta$,we have $\cos \phi = \sin \theta$ and $\cot \phi = \tan \theta$.
So the equations are:
$1) x \cos \theta + y \cot \theta = 3$
$2) x \sin \theta + y \tan \theta = 3$
Solving for $y = \beta$ by eliminating $x$: Multiply $(1)$ by $\sin \theta$ and $(2)$ by $\cos \theta$:
$x \cos \theta \sin \theta + y \cot \theta \sin \theta = 3 \sin \theta$
$x \sin \theta \cos \theta + y \tan \theta \cos \theta = 3 \cos \theta$
Subtracting the two equations:
$y(\cos \theta - \sin \theta) = 3(\sin \theta - \cos \theta)$
$y = -3$
Thus,$\beta = -3$. Then $(2\beta)^2 = (2 \times -3)^2 = (-6)^2 = 36$.
283
DifficultMCQ
The locus of the centroid of the triangle formed by any point $P$ on the hyperbola $16x^{2}-9y^{2}+32x+36y-164=0$ and its foci is:
A
$9x^{2}-16y^{2}+36x+32y-36=0$
B
$16x^{2}-9y^{2}+32x+36y-36=0$
C
$16x^{2}-9y^{2}+32x+36y-144=0$
D
$9x^{2}-16y^{2}+36x+32y-144=0$

Solution

(B) The given hyperbola is $16(x+1)^{2}-9(y-2)^{2}=164+16-36=144$.
This simplifies to $\frac{(x+1)^{2}}{9}-\frac{(y-2)^{2}}{16}=1$.
The center of the hyperbola is $(-1, 2)$.
Here $a^{2}=9$ and $b^{2}=16$,so $e=\sqrt{1+\frac{16}{9}}=\frac{5}{3}$.
The foci are $(h \pm ae, k) = (-1 \pm 3 \times \frac{5}{3}, 2) = (-1 \pm 5, 2)$,which are $(4, 2)$ and $(-6, 2)$.
Let $P(\alpha, \beta)$ be a point on the hyperbola.
Let $G(x, y)$ be the centroid of the triangle formed by $P(\alpha, \beta)$,$(4, 2)$,and $(-6, 2)$.
Then $x=\frac{\alpha+4-6}{3} = \frac{\alpha-2}{3} \Rightarrow \alpha=3x+2$.
And $y=\frac{\beta+2+2}{3} = \frac{\beta+4}{3} \Rightarrow \beta=3y-4$.
Since $P(\alpha, \beta)$ lies on the hyperbola $16(x+1)^{2}-9(y-2)^{2}=144$,we substitute $\alpha$ and $\beta$:
$16(3x+2+1)^{2}-9(3y-4-2)^{2}=144$
$16(3x+3)^{2}-9(3y-6)^{2}=144$
$16 \times 9(x+1)^{2}-9 \times 9(y-2)^{2}=144$
$144(x+1)^{2}-81(y-2)^{2}=144$
Divide by $9$:
$16(x+1)^{2}-9(y-2)^{2}=16$
$16(x^{2}+2x+1)-9(y^{2}-4y+4)=16$
$16x^{2}+32x+16-9y^{2}+36y-36=16$
$16x^{2}-9y^{2}+32x+36y-36=0$.
Solution diagram
284
DifficultMCQ
Let $\lambda x - 2y = \mu$ be a tangent to the hyperbola $a^{2}x^{2} - y^{2} = b^{2}$. Then $\left(\frac{\lambda}{a}\right)^{2} - \left(\frac{\mu}{b}\right)^{2}$ is equal to
A
$-2$
B
$-4$
C
$2$
D
$4$

Solution

(D) The given hyperbola is $a^{2}x^{2} - y^{2} = b^{2}$,which can be written as $\frac{x^{2}}{(b/a)^{2}} - \frac{y^{2}}{b^{2}} = 1$.
For a line $y = mx + c$ to be a tangent to the hyperbola $\frac{x^{2}}{A^{2}} - \frac{y^{2}}{B^{2}} = 1$,the condition is $c^{2} = A^{2}m^{2} - B^{2}$.
Here,the line is $\lambda x - 2y = \mu$,which can be rewritten as $y = \frac{\lambda}{2}x - \frac{\mu}{2}$.
Comparing with $y = mx + c$,we have $m = \frac{\lambda}{2}$ and $c = -\frac{\mu}{2}$.
Here $A^{2} = \frac{b^{2}}{a^{2}}$ and $B^{2} = b^{2}$.
Substituting these into the condition $c^{2} = A^{2}m^{2} - B^{2}$:
$(-\frac{\mu}{2})^{2} = \frac{b^{2}}{a^{2}}(\frac{\lambda}{2})^{2} - b^{2}$
$\frac{\mu^{2}}{4} = \frac{b^{2}\lambda^{2}}{4a^{2}} - b^{2}$
Multiply by $\frac{4}{b^{2}}$:
$\frac{\mu^{2}}{b^{2}} = \frac{\lambda^{2}}{a^{2}} - 4$
Rearranging the terms,we get $\frac{\lambda^{2}}{a^{2}} - \frac{\mu^{2}}{b^{2}} = 4$.
285
MediumMCQ
Let the eccentricity of the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ be $\frac{5}{4}$. If the equation of the normal at the point $\left(\frac{8}{\sqrt{5}}, \frac{12}{5}\right)$ on the hyperbola is $8 \sqrt{5} x + \beta y = \lambda$,then $\lambda - \beta$ is equal to
A
$89$
B
$85$
C
$78$
D
$45$

Solution

(B) Given eccentricity $e = \frac{5}{4}$,so $e^{2} = 1 + \frac{b^{2}}{a^{2}} = \frac{25}{16}$ $\Rightarrow \frac{b^{2}}{a^{2}} = \frac{9}{16}$ $\Rightarrow b^{2} = \frac{9}{16}a^{2}$.
The point $\left(\frac{8}{\sqrt{5}}, \frac{12}{5}\right)$ lies on the hyperbola,so $\frac{64}{5a^{2}} - \frac{144}{25b^{2}} = 1$.
Substituting $b^{2} = \frac{9}{16}a^{2}$,we get $\frac{64}{5a^{2}} - \frac{144}{25 \times (9/16)a^{2}} = 1$ $\Rightarrow \frac{64}{5a^{2}} - \frac{144 \times 16}{225a^{2}} = 1$ $\Rightarrow \frac{64}{5a^{2}} - \frac{256}{25a^{2}} = 1$.
$\frac{320 - 256}{25a^{2}} = 1$ $\Rightarrow \frac{64}{25a^{2}} = 1$ $\Rightarrow a^{2} = \frac{64}{25}$ $\Rightarrow a = \frac{8}{5}$.
Then $b^{2} = \frac{9}{16} \times \frac{64}{25} = \frac{9 \times 4}{25} = \frac{36}{25} \Rightarrow b = \frac{6}{5}$.
The equation of the normal at $(x_{1}, y_{1})$ is $\frac{a^{2}x}{x_{1}} + \frac{b^{2}y}{y_{1}} = a^{2} + b^{2}$.
Substituting values: $\frac{(64/25)x}{8/\sqrt{5}} + \frac{(36/25)y}{12/5} = \frac{64}{25} + \frac{36}{25} = \frac{100}{25} = 4$.
$\frac{8\sqrt{5}}{25}x + \frac{3}{5}y = 4 \Rightarrow 8\sqrt{5}x + 15y = 100$.
Comparing with $8\sqrt{5}x + \beta y = \lambda$,we get $\beta = 15$ and $\lambda = 100$.
Thus,$\lambda - \beta = 100 - 15 = 85$.
286
MediumMCQ
The normal to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{9}=1$ at the point $(8, 3\sqrt{3})$ on it passes through the point
A
$(15, -2\sqrt{3})$
B
$(9, 2\sqrt{3})$
C
$(-1, 9\sqrt{3})$
D
$(-1, 6\sqrt{3})$

Solution

(C) Given the hyperbola equation $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{9}=1$.
Since the point $(8, 3\sqrt{3})$ lies on the hyperbola,we substitute the coordinates into the equation:
$\frac{64}{a^{2}}-\frac{(3\sqrt{3})^{2}}{9}=1$
$\frac{64}{a^{2}}-\frac{27}{9}=1$
$\frac{64}{a^{2}}-3=1$ $\Rightarrow \frac{64}{a^{2}}=4$ $\Rightarrow a^{2}=16$.
The equation of the normal to the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ at point $(x_{1}, y_{1})$ is given by $\frac{a^{2}x}{x_{1}}+\frac{b^{2}y}{y_{1}}=a^{2}+b^{2}$.
Substituting $a^{2}=16, b^{2}=9, x_{1}=8, y_{1}=3\sqrt{3}$:
$\frac{16x}{8}+\frac{9y}{3\sqrt{3}}=16+9$
$2x+\sqrt{3}y=25$.
Checking the options:
For option $A$: $2(15)+\sqrt{3}(-2\sqrt{3}) = 30-6 = 24 \neq 25$.
For option $B$: $2(9)+\sqrt{3}(2\sqrt{3}) = 18+6 = 24 \neq 25$.
For option $C$: $2(-1)+\sqrt{3}(9\sqrt{3}) = -2+27 = 25$.
Thus,the normal passes through the point $(-1, 9\sqrt{3})$.
287
DifficultMCQ
Let a line $L_{1}$ be tangent to the hyperbola $\frac{x^{2}}{16}-\frac{y^{2}}{4}=1$ and let $L_{2}$ be the line passing through the origin and perpendicular to $L_{1}$. If the locus of the point of intersection of $L_{1}$ and $L_{2}$ is $(x^{2}+y^{2})^{2} = \alpha x^{2}+\beta y^{2}$,then $\alpha+\beta$ is equal to
A
$11$
B
$12$
C
$15$
D
$16$

Solution

(B) The equation of the tangent $L_{1}$ to the hyperbola $\frac{x^{2}}{16}-\frac{y^{2}}{4}=1$ at point $(4 \sec \theta, 2 \tan \theta)$ is $\frac{x \sec \theta}{4}-\frac{y \tan \theta}{2}=1$.
The slope of $L_{1}$ is $m_{1} = \frac{\sec \theta}{2 \tan \theta}$.
Let the point of intersection be $(h, k)$. Since $L_{2}$ passes through $(0, 0)$ and $(h, k)$,its slope is $m_{2} = \frac{k}{h}$.
Since $L_{1} \perp L_{2}$,we have $m_{1} m_{2} = -1$,so $\frac{k}{h} \cdot \frac{\sec \theta}{2 \tan \theta} = -1$,which gives $\frac{k}{2h \sin \theta} = -1$,or $\sin \theta = -\frac{k}{2h}$.
Then $\cos^{2} \theta = 1 - \sin^{2} \theta = 1 - \frac{k^{2}}{4h^{2}} = \frac{4h^{2}-k^{2}}{4h^{2}}$,so $\cos \theta = \frac{\sqrt{4h^{2}-k^{2}}}{2h}$.
Substituting these into the tangent equation $\frac{h \sec \theta}{4} - \frac{k \tan \theta}{2} = 1$:
$\frac{h}{4} \cdot \frac{2h}{\sqrt{4h^{2}-k^{2}}} - \frac{k}{2} \cdot \left( \frac{-k}{\sqrt{4h^{2}-k^{2}}} \right) = 1$
$\frac{2h^{2}+k^{2}}{2\sqrt{4h^{2}-k^{2}}} = 1 \implies 2h^{2}+k^{2} = 2\sqrt{4h^{2}-k^{2}}$
Squaring both sides: $(2h^{2}+k^{2})^{2} = 4(4h^{2}-k^{2}) = 16h^{2}-4k^{2}$.
Replacing $(h, k)$ with $(x, y)$,we get $(2x^{2}+y^{2})^{2} = 16x^{2}-4y^{2}$.
Wait,the standard locus of the foot of the perpendicular from the origin to a tangent of a hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ is $(x^{2}+y^{2})^{2} = a^{2}x^{2}-b^{2}y^{2}$.
Here $a^{2}=16$ and $b^{2}=4$,so the locus is $(x^{2}+y^{2})^{2} = 16x^{2}-4y^{2}$.
Thus $\alpha=16$ and $\beta=-4$.
$\alpha+\beta = 16-4 = 12$.
Solution diagram
288
DifficultMCQ
Let $a>0, b>0$. Let $e$ and $\ell$ respectively be the eccentricity and length of the latus rectum of the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$. Let $e^{\prime}$ and $\ell^{\prime}$ respectively be the eccentricity and length of the latus rectum of its conjugate hyperbola. If $e^{2}=\frac{11}{14} \ell$ and $(e^{\prime})^{2}=\frac{11}{8} \ell^{\prime}$,then the value of $77a+44b$ is equal to
A
$100$
B
$110$
C
$120$
D
$130$

Solution

(D) For the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,we have $e=\sqrt{1+\frac{b^{2}}{a^{2}}}$ and $\ell=\frac{2b^{2}}{a}$.
Given $e^{2}=\frac{11}{14}\ell$,we get $1+\frac{b^{2}}{a^{2}}=\frac{11}{14} \cdot \frac{2b^{2}}{a}$,which simplifies to $\frac{a^{2}+b^{2}}{a^{2}}=\frac{11b^{2}}{7a} \dots (1)$.
For the conjugate hyperbola $\frac{y^{2}}{b^{2}}-\frac{x^{2}}{a^{2}}=1$,we have $e^{\prime}=\sqrt{1+\frac{a^{2}}{b^{2}}}$ and $\ell^{\prime}=\frac{2a^{2}}{b}$.
Given $(e^{\prime})^{2}=\frac{11}{8}\ell^{\prime}$,we get $1+\frac{a^{2}}{b^{2}}=\frac{11}{8} \cdot \frac{2a^{2}}{b}$,which simplifies to $\frac{a^{2}+b^{2}}{b^{2}}=\frac{11a^{2}}{4b} \dots (2)$.
Dividing $(1)$ by $(2)$,we get $\frac{b^{2}}{a^{2}}=\frac{11b^{2}}{7a} \cdot \frac{4b}{11a^{2}} = \frac{4b^{3}}{7a^{3}}$.
Thus,$7a^{3}=4b^{3} \cdot \frac{a^{2}}{b^{2}} \implies 7a=4b \dots (3)$.
Substituting $a=\frac{4b}{7}$ into $(2)$: $\frac{(\frac{16b^{2}}{49})+b^{2}}{b^{2}} = \frac{11}{4} \cdot \frac{16b^{2}}{49b} \implies \frac{65}{49} = \frac{11 \cdot 4b}{49} \implies 65 = 44b \implies b = \frac{65}{44}$.
Then $a = \frac{4}{7} \cdot \frac{65}{44} = \frac{65}{77}$.
Finally,$77a+44b = 77(\frac{65}{77}) + 44(\frac{65}{44}) = 65+65 = 130$.
289
DifficultMCQ
Let the eccentricity of the hyperbola $H : \frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ be $\sqrt{\frac{5}{2}}$ and the length of its latus rectum be $6\sqrt{2}$. If $y = 2x + c$ is a tangent to the hyperbola $H$,then the value of $c^2$ is equal to
A
$18$
B
$20$
C
$24$
D
$32$

Solution

(B) The condition for the line $y = mx + c$ to be a tangent to the hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ is $c^2 = a^2m^2 - b^2$.
Given $m = 2$,we have $c^2 = 4a^2 - b^2$.
The eccentricity $e$ is given by $e^2 = 1 + \frac{b^2}{a^2}$.
Substituting $e^2 = \frac{5}{2}$,we get $\frac{5}{2} = 1 + \frac{b^2}{a^2}$,which implies $\frac{b^2}{a^2} = \frac{3}{2}$,so $b^2 = \frac{3a^2}{2}$.
The length of the latus rectum is $\frac{2b^2}{a} = 6\sqrt{2}$.
Substituting $b^2 = \frac{3a^2}{2}$,we get $\frac{2}{a} \times \frac{3a^2}{2} = 6\sqrt{2}$,which simplifies to $3a = 6\sqrt{2}$,so $a = 2\sqrt{2}$.
Then $a^2 = 8$ and $b^2 = \frac{3}{2} \times 8 = 12$.
Finally,$c^2 = 4a^2 - b^2 = 4(8) - 12 = 32 - 12 = 20$.
290
AdvancedMCQ
Let $H : \frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$,$a > 0, b > 0$,be a hyperbola such that the sum of lengths of the transverse and the conjugate axes is $4(2\sqrt{2}+\sqrt{14})$. If the eccentricity of $H$ is $\frac{\sqrt{11}}{2}$,then the value of $a^{2}+b^{2}$ is equal to
A
$89$
B
$90$
C
$87$
D
$88$

Solution

(D) The equation of the hyperbola is $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$.
Given the eccentricity $e = \frac{\sqrt{11}}{2}$,we have $e^{2} = 1 + \frac{b^{2}}{a^{2}}$.
Substituting the value of $e$,we get $\frac{11}{4} = 1 + \frac{b^{2}}{a^{2}}$,which implies $\frac{b^{2}}{a^{2}} = \frac{7}{4}$,so $b = \frac{\sqrt{7}}{2}a$.
The sum of the lengths of the transverse axis $(2a)$ and the conjugate axis $(2b)$ is given as $2a + 2b = 4(2\sqrt{2} + \sqrt{14})$.
Substituting $b = \frac{\sqrt{7}}{2}a$,we get $2a + 2(\frac{\sqrt{7}}{2}a) = 4(2\sqrt{2} + \sqrt{14})$.
$2a + \sqrt{7}a = 4(2\sqrt{2} + \sqrt{2}\sqrt{7})$.
$a(2 + \sqrt{7}) = 4\sqrt{2}(2 + \sqrt{7})$.
Thus,$a = 4\sqrt{2}$,which means $a^{2} = 16 \times 2 = 32$.
Since $b^{2} = \frac{7}{4}a^{2}$,we have $b^{2} = \frac{7}{4} \times 32 = 7 \times 8 = 56$.
Therefore,$a^{2} + b^{2} = 32 + 56 = 88$.
291
AdvancedMCQ
Let the equation of two diameters of a circle $x^{2} + y^{2} - 2x + 2fy + 1 = 0$ be $2px - y = 1$ and $2x + py = 4p$. Then the slope $m \in (0, \infty)$ of the tangent to the hyperbola $3x^{2} - y^{2} = 3$ passing through the centre of the circle is equal to $......$
A
$6$
B
$2$
C
$4$
D
$8$

Solution

(B) The equation of the circle is $x^{2} + y^{2} - 2x + 2fy + 1 = 0$. The centre of the circle is $(1, -f)$.
Since the diameters pass through the centre,we substitute $(1, -f)$ into the equations of the diameters:
$2p(1) - (-f) = 1 \implies 2p + f = 1 \implies f = 1 - 2p$.
$2(1) + p(-f) = 4p \implies 2 - pf = 4p$.
Substituting $f = 1 - 2p$ into the second equation:
$2 - p(1 - 2p) = 4p \implies 2 - p + 2p^{2} = 4p \implies 2p^{2} - 5p + 2 = 0$.
Solving for $p$: $(2p - 1)(p - 2) = 0$,so $p = 1/2$ or $p = 2$.
If $p = 1/2$,$f = 1 - 2(1/2) = 0$. Centre is $(1, 0)$.
If $p = 2$,$f = 1 - 2(2) = -3$. Centre is $(1, 3)$.
The hyperbola is $3x^{2} - y^{2} = 3$,which is $x^{2} - y^{2}/3 = 1$. The tangent line $y = mx + c$ has $c^{2} = a^{2}m^{2} - b^{2} = m^{2} - 3$.
Case $1$: Centre $(1, 0)$. $0 = m(1) + c \implies c = -m$. Then $c^{2} = m^{2} = m^{2} - 3$,which is impossible.
Case $2$: Centre $(1, 3)$. $3 = m(1) + c \implies c = 3 - m$. Then $c^{2} = (3 - m)^{2} = m^{2} - 3$.
$9 - 6m + m^{2} = m^{2} - 3 \implies 6m = 12 \implies m = 2$.
292
MediumMCQ
If the line $x-1=0$ is a directrix of the hyperbola $kx^{2}-y^{2}=6$,then the hyperbola passes through which of the following points?
A
$(-2\sqrt{5}, 6)$
B
$(-\sqrt{5}, 3)$
C
$(\sqrt{5}, -2)$
D
$(2\sqrt{5}, 3\sqrt{6})$

Solution

(C) The given equation of the hyperbola is $kx^{2}-y^{2}=6$,which can be written as $\frac{x^{2}}{6/k} - \frac{y^{2}}{6} = 1$.
Here,$a^{2} = \frac{6}{k}$ and $b^{2} = 6$.
The eccentricity $e$ is given by $e^{2} = 1 + \frac{b^{2}}{a^{2}} = 1 + \frac{6}{6/k} = 1 + k$.
Thus,$e = \sqrt{1+k}$.
The equation of the directrix for a hyperbola $\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1$ is $x = \frac{a}{e}$.
Given the directrix is $x = 1$,we have $1 = \frac{\sqrt{6/k}}{\sqrt{1+k}}$.
Squaring both sides,$1 = \frac{6}{k(1+k)} \Rightarrow k^{2} + k - 6 = 0$.
Solving the quadratic equation $(k+3)(k-2) = 0$,we get $k = 2$ (since $k$ must be positive for a hyperbola).
The equation of the hyperbola is $2x^{2} - y^{2} = 6$.
Checking the options: For option $C$,$2(\sqrt{5})^{2} - (-2)^{2} = 2(5) - 4 = 10 - 4 = 6$. Thus,the point $(\sqrt{5}, -2)$ lies on the hyperbola.
293
MediumMCQ
Let the tangent drawn to the parabola $y^2 = 24x$ at the point $(\alpha, \beta)$ be perpendicular to the line $2x + 2y = 5$. Then the normal to the hyperbola $\frac{x^2}{\alpha^2} - \frac{y^2}{\beta^2} = 1$ at the point $(\alpha + 4, \beta + 4)$ does $NOT$ pass through which of the following points?
A
$(25, 10)$
B
$(20, 12)$
C
$(30, 8)$
D
$(15, 13)$

Solution

(D) The tangent to the parabola $y^2 = 24x$ at $(\alpha, \beta)$ is given by $y\beta = 12(x + \alpha)$. The slope of this tangent is $m_1 = \frac{12}{\beta}$.
Given the line $2x + 2y = 5$,its slope is $m_2 = -1$. Since the tangent is perpendicular to this line,$m_1 \times m_2 = -1$,so $\frac{12}{\beta} \times (-1) = -1$,which gives $\beta = 12$.
Since $(\alpha, \beta)$ lies on $y^2 = 24x$,we have $12^2 = 24\alpha$,so $144 = 24\alpha$,which gives $\alpha = 6$.
The hyperbola is $\frac{x^2}{36} - \frac{y^2}{144} = 1$. The point on the hyperbola is $(\alpha + 4, \beta + 4) = (10, 16)$.
The equation of the normal to the hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ at $(x_0, y_0)$ is $\frac{a^2x}{x_0} + \frac{b^2y}{y_0} = a^2 + b^2$.
Here $a^2 = 36, b^2 = 144, x_0 = 10, y_0 = 16$. The normal is $\frac{36x}{10} + \frac{144y}{16} = 36 + 144$,which simplifies to $3.6x + 9y = 180$,or $2x + 5y = 100$.
Checking the options: For $(25, 10)$,$2(25) + 5(10) = 50 + 50 = 100$ (Passes).
For $(20, 12)$,$2(20) + 5(12) = 40 + 60 = 100$ (Passes).
For $(30, 8)$,$2(30) + 5(8) = 60 + 40 = 100$ (Passes).
For $(15, 13)$,$2(15) + 5(13) = 30 + 65 = 95 \neq 100$ (Does not pass).
294
AdvancedMCQ
On a rectangular hyperbola $x^2-y^2=a^2, a > 0$,three points $A, B, C$ are taken as follows: $A=(-a, 0)$; $B$ and $C$ are placed symmetrically with respect to the $X$-axis on the branch of the hyperbola not containing $A$. Suppose that the $\triangle ABC$ is equilateral. If the side length of the $\triangle ABC$ is $ka$,then $k$ lies in the interval
A
$(0, 2]$
B
$(2, 4]$
C
$(4, 6]$
D
$(6, 8]$

Solution

(B) The rectangular hyperbola is given by $x^2-y^2=a^2$.
Let the coordinates of $B$ be $(a \sec \theta, a \tan \theta)$ and $C$ be $(a \sec \theta, -a \tan \theta)$.
Since $\triangle ABC$ is equilateral,$AB^2 = BC^2$.
The distance $BC = 2a \tan \theta$.
The distance $AB^2 = (a \sec \theta - (-a))^2 + (a \tan \theta - 0)^2 = a^2(\sec \theta + 1)^2 + a^2 \tan^2 \theta$.
Equating $AB^2 = BC^2$:
$a^2(\sec \theta + 1)^2 + a^2 \tan^2 \theta = (2a \tan \theta)^2 = 4a^2 \tan^2 \theta$.
$(\sec \theta + 1)^2 = 3 \tan^2 \theta = 3(\sec^2 \theta - 1) = 3(\sec \theta + 1)(\sec \theta - 1)$.
Since $\sec \theta + 1 \neq 0$,we have $\sec \theta + 1 = 3(\sec \theta - 1)$.
$\sec \theta + 1 = 3 \sec \theta - 3 \implies 2 \sec \theta = 4 \implies \sec \theta = 2$.
Thus,$\tan^2 \theta = \sec^2 \theta - 1 = 4 - 1 = 3$,so $\tan \theta = \sqrt{3}$.
The side length $BC = 2a \tan \theta = 2a \sqrt{3}$.
Given the side length is $ka$,we have $ka = 2a \sqrt{3}$,so $k = 2 \sqrt{3} \approx 2 \times 1.732 = 3.464$.
Since $2 < 3.464 \leq 4$,$k$ lies in the interval $(2, 4]$.
Solution diagram
295
AdvancedMCQ
The sound of a cannon firing is heard one second later at position $B$ than at position $A$. If the speed of sound is uniform,then
A
the positions $A$ and $B$ are foci of a hyperbola,with the cannon's position on one branch of the hyperbola
B
the positions $A$ and $B$ are foci of an ellipse with the cannon's position on the ellipse
C
one of the positions $A, B$ is a focus of a parabola with the cannon's position on the parabola
D
it is not possible to describe the positions of $A, B$ and the cannon with the given information

Solution

(A) Let the position of the cannon be $P$ and the speed of sound be $S$.
Let the time at which the sound of the cannon firing is heard at $A$ be $t$.
Therefore,the time at which it is heard at $B$ is $t+1$.
Hence,the distance $PA = \text{Speed} \times \text{Time} = St$.
Similarly,the distance $PB = S(t+1) = St + S$.
Subtracting the two distances,we get $PB - PA = (St + S) - St = S$.
Since $S$ is the speed of sound (a constant),$PB - PA = \text{constant}$.
By the definition of a hyperbola,the locus of a point such that the difference of its distances from two fixed points (foci) is constant is a hyperbola.
Thus,$A$ and $B$ are the foci of a hyperbola,and the cannon's position $P$ lies on one branch of this hyperbola.
Solution diagram
296
AdvancedMCQ
$A$ mirror in the first quadrant is in the shape of a hyperbola whose equation is $xy=1$. $A$ light source in the second quadrant emits a beam of light that hits the mirror at the point $(2, 1/2)$. If the reflected ray is parallel to the $Y$-axis,the slope of the incident beam is
A
$\frac{13}{8}$
B
$\frac{7}{4}$
C
$\frac{15}{8}$
D
$2$

Solution

(C) The equation of the hyperbola is $xy=1$,which can be written as $y = \frac{1}{x}$.
Differentiating with respect to $x$,we get $\frac{dy}{dx} = -\frac{1}{x^2}$.
The slope of the tangent at the point $(2, 1/2)$ is $\left(\frac{dy}{dx}\right)_{(2, 1/2)} = -\frac{1}{2^2} = -\frac{1}{4}$.
The slope of the normal at $(2, 1/2)$ is $n = -\frac{1}{(-1/4)} = 4$.
Let $m$ be the slope of the incident beam of light. The reflected ray is parallel to the $Y$-axis,so its slope is $\infty$.
According to the law of reflection,the angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal. Using the formula for the angle between two lines with slopes $m_1$ and $m_2$,$\tan \theta = \left|\frac{m_1 - m_2}{1 + m_1 m_2}\right|$,we have:
$\left|\frac{4 - m}{1 + 4m}\right| = \left|\frac{\infty - 4}{1 + 4(\infty)}\right| = \left|\frac{1}{4}\right|$.
Solving $\frac{4 - m}{1 + 4m} = \frac{1}{4}$:
$16 - 4m = 1 + 4m$
$8m = 15 \Rightarrow m = \frac{15}{8}$.
Solving $\frac{4 - m}{1 + 4m} = -\frac{1}{4}$:
$16 - 4m = -1 - 4m$
$16 = -1$,which is impossible.
Thus,the slope of the incident beam is $\frac{15}{8}$.
Solution diagram
297
DifficultMCQ
The vertices of a hyperbola $H$ are $(\pm 6, 0)$ and its eccentricity is $\frac{\sqrt{5}}{2}$. Let $N$ be the normal to $H$ at a point in the first quadrant and parallel to the line $\sqrt{2} x + y = 2 \sqrt{2}$. If $d$ is the length of the line segment of $N$ between $H$ and the $y$-axis,then $d^2$ is equal to $............$.
A
$215$
B
$216$
C
$217$
D
$218$

Solution

(B) Given the vertices of the hyperbola are $(\pm 6, 0)$,we have $a = 6$.
Given eccentricity $e = \frac{\sqrt{5}}{2}$.
We know $b^2 = a^2(e^2 - 1) = 36 \left( \frac{5}{4} - 1 \right) = 36 \left( \frac{1}{4} \right) = 9$.
So,the equation of the hyperbola $H$ is $\frac{x^2}{36} - \frac{y^2}{9} = 1$.
The equation of the normal to the hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ at point $(a \sec \theta, b \tan \theta)$ is $\frac{ax}{\sec \theta} + \frac{by}{\tan \theta} = a^2 + b^2$.
Substituting $a=6, b=3$,the equation is $\frac{6x}{\sec \theta} + \frac{3y}{\tan \theta} = 36 + 9 = 45$,which simplifies to $6x \cos \theta + 3y \cot \theta = 45$.
The slope of this normal is $-\frac{6 \cos \theta}{3 \cot \theta} = -2 \sin \theta$.
The normal is parallel to $\sqrt{2}x + y = 2\sqrt{2}$,which has a slope of $-\sqrt{2}$.
So,$-2 \sin \theta = -\sqrt{2} \Rightarrow \sin \theta = \frac{1}{\sqrt{2}} \Rightarrow \theta = \frac{\pi}{4}$.
Substituting $\theta = \frac{\pi}{4}$ into the normal equation: $6x \cos(\frac{\pi}{4}) + 3y \cot(\frac{\pi}{4}) = 45 \Rightarrow 6x(\frac{1}{\sqrt{2}}) + 3y(1) = 45 \Rightarrow 3\sqrt{2}x + 3y = 45 \Rightarrow \sqrt{2}x + y = 15$.
The point $P$ on the hyperbola is $(6 \sec(\frac{\pi}{4}), 3 \tan(\frac{\pi}{4})) = (6\sqrt{2}, 3)$.
The intersection of the normal with the $y$-axis $(x=0)$ is $K(0, 15)$.
The length $d$ is the distance between $P(6\sqrt{2}, 3)$ and $K(0, 15)$.
$d^2 = (6\sqrt{2} - 0)^2 + (3 - 15)^2 = (6\sqrt{2})^2 + (-12)^2 = 72 + 144 = 216$.
Solution diagram
298
DifficultMCQ
Let $H$ be the hyperbola,whose foci are $(1 \pm \sqrt{2}, 0)$ and eccentricity is $e = \sqrt{2}$. Then the length of its latus rectum is
A
$2$
B
$3$
C
$\frac{5}{2}$
D
$\frac{3}{2}$

Solution

(A) The foci are given by $(h \pm ae, k) = (1 \pm \sqrt{2}, 0)$.
Comparing,we get the center $(h, k) = (1, 0)$ and $ae = \sqrt{2}$.
Given the eccentricity $e = \sqrt{2}$,we have $a(\sqrt{2}) = \sqrt{2}$,which implies $a = 1$.
For a hyperbola,$b^2 = a^2(e^2 - 1)$.
Substituting the values,$b^2 = 1^2((\sqrt{2})^2 - 1) = 1(2 - 1) = 1$.
Thus,$b = 1$.
The length of the latus rectum is given by $\frac{2b^2}{a} = \frac{2(1)^2}{1} = 2$.
299
DifficultMCQ
Let $P(x_0, y_0)$ be the point on the hyperbola $3x^2 - 4y^2 = 36$ which is nearest to the line $3x + 2y = 1$. Then $\sqrt{2}(y_0 - x_0)$ is equal to:
A
$-3$
B
$9$
C
$-9$
D
$3$

Solution

(C) The equation of the hyperbola is $3x^2 - 4y^2 = 36$,which can be written as $\frac{x^2}{12} - \frac{y^2}{9} = 1$.
Here $a^2 = 12$ and $b^2 = 9$,so $a = 2\sqrt{3}$ and $b = 3$.
The parametric coordinates of a point $P$ on the hyperbola are $(a \sec \theta, b \tan \theta) = (2\sqrt{3} \sec \theta, 3 \tan \theta)$.
The slope of the tangent at $P$ must be equal to the slope of the line $3x + 2y = 1$,which is $m = -\frac{3}{2}$.
The slope of the tangent at $(x_0, y_0)$ is $\frac{dy}{dx} = \frac{3x_0}{4y_0} = -\frac{3}{2} \implies \frac{x_0}{2y_0} = -1 \implies x_0 = -2y_0$.
Substitute $x_0 = -2y_0$ into the hyperbola equation: $3(-2y_0)^2 - 4y_0^2 = 36 \implies 12y_0^2 - 4y_0^2 = 36 \implies 8y_0^2 = 36 \implies y_0^2 = \frac{9}{2} \implies y_0 = \pm \frac{3}{\sqrt{2}}$.
Since the line $3x + 2y = 1$ is in the first and fourth quadrants,we check the distance. For $y_0 = -\frac{3}{\sqrt{2}}$,$x_0 = -2(-\frac{3}{\sqrt{2}}) = \frac{6}{\sqrt{2}} = 3\sqrt{2}$.
Then $\sqrt{2}(y_0 - x_0) = \sqrt{2}(-\frac{3}{\sqrt{2}} - \frac{6}{\sqrt{2}}) = -3 - 6 = -9$.
300
DifficultMCQ
Let $R$ be a rectangle given by the lines $x=0, x=2, y=0$ and $y=5$. Let $A(\alpha, 0)$ and $B(0, \beta)$,where $\alpha \in [0, 2]$ and $\beta \in [0, 5]$,be such that the line segment $AB$ divides the area of the rectangle $R$ in the ratio $4:1$. Then,the mid-point of $AB$ lies on a $.........$.
A
parabola
B
hyperbola
C
straight line
D
circle

Solution

(B) The area of the rectangle $R$ is $2 \times 5 = 10$ square units.
The line segment $AB$ cuts off a triangle $OAB$ from the rectangle,where $O$ is the origin $(0,0)$.
The area of triangle $OAB$ is $\frac{1}{2} \times \alpha \times \beta = \frac{\alpha \beta}{2}$.
The line segment $AB$ divides the rectangle into two parts with areas in the ratio $4:1$. The area of the triangle $OAB$ is the smaller part,so its area must be $\frac{1}{5}$ of the total area of the rectangle.
$\frac{\text{Area}(OAB)}{\text{Area}(R)} = \frac{1}{5} \implies \frac{\alpha \beta / 2}{10} = \frac{1}{5} \implies \frac{\alpha \beta}{20} = \frac{1}{5} \implies \alpha \beta = 4$.
Let $M(h, k)$ be the mid-point of $AB$. Then $h = \frac{\alpha}{2}$ and $k = \frac{\beta}{2}$,which means $\alpha = 2h$ and $\beta = 2k$.
Substituting these into $\alpha \beta = 4$,we get $(2h)(2k) = 4 \implies 4hk = 4 \implies hk = 1$.
The locus of the mid-point $M(x, y)$ is $xy = 1$,which represents a hyperbola.
Solution diagram

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