Ex 5.6
Choose the correct or the most suitable answer from the given four alternatives.
Question 1.
The equation of the circle passing through $(1,5)$ and $(4,1)$ and touching $y$-axis is $x^2+y^2-5 x-6 y+9+\lambda$ $(4 x+3 y-19)=0$ where $\lambda$ is equal to ..........
(a) $0,-\frac{40}{9}$
(b) 0
(c) $\frac{40}{9}$
(d) $-\frac{40}{9}$
Solution:
(a) $0,-\frac{40}{9}$
Hint:
$
\begin{aligned}
& x^2+y^2-5 x-6 y+9+\lambda(4 x+3 y-19)=0 \\
& \text { ie., } x^2+y^2+x(4 \lambda-5)+y(3 \lambda-6)+9-19 \lambda=0
\end{aligned}
$
Since it touches $y$ axis $\mathrm{x}=0$
$
\Rightarrow y^2+y(3 \lambda-6)+9-19 \lambda=0
$
It is a quadratic in $\mathrm{y}$,
Since the circle touches $y$ axis the roots must be equal $\Rightarrow b^2-4 a c=0$
ie., $(3 \lambda-6)^2-4(1)(9-19 \lambda)=0$
Solving we get $\lambda=0$ or $-\frac{40}{9}$
Question 2.
The eccentricity of the hyperbola whose latus rectum is 8 and conjugate axis is equal to half the distance between the foci is.
(a) $\frac{4}{3}$
(b) $\frac{4}{\sqrt{3}}$
(c) $\frac{2}{\sqrt{3}}$
(d) $\frac{3}{2}$
Solution:
(c) $\frac{2}{\sqrt{3}}$
Hint:
Given $\frac{2 b^2}{a}=8$ and $2 \mathrm{~b}=\mathrm{ae}$
$
\begin{array}{rlrl}
\Rightarrow \quad b & =\frac{a e}{2} \quad & \Rightarrow b^2=\frac{a^2 e^2}{4} \\
\text { but } b^2 & =a^2\left(e^2-1\right)=a^2 e^2-a^2 \\
a^2 e^2-a^2 & =\frac{a^2 e^2}{4} \quad & \Rightarrow a^2 e^2-\frac{a^2 e^2}{4}=a^2 \\
\frac{3 a^2 e^2}{4} & =a^2 \\
e & =\sqrt{\frac{4}{3}}=\frac{2}{\sqrt{3}} &
\end{array}
$
Question 3.
The circle $x^2+y^2=4 x+8 y+5$ intersects the line $3 x-4 y=m$ at two distinct points if
(a) $15<\mathrm{m}<65$
(b) $35<\mathrm{m}<85$
(c) $-85<\mathrm{m}<-35$
(d) $-35<$ m $<15$
Solution:
(d) $-35<\mathrm{m}<15$
Hint:
$x^2+y^2-4 x-8 y-5=0$
$3 x-4 y=m$ Solving (1) and (2) we get
from (2) $\Rightarrow 3 x=4 y+m$
$\mathrm{x}=\frac{4 y+m}{3}$
Substituting $\mathrm{x}$ value in (1) we get
$
\left(\frac{4 y+m}{3}\right)^2+y^2-4\left(\frac{4 y+m}{3}\right)-8 y-5=0
$
It is a quadratic in $y$ and given that the roots are distinct
$
\Rightarrow b^2-4 a c>0
$
On simplifying we get
$
\begin{aligned}
& \Rightarrow-9 \mathrm{~m}^2-18 \mathrm{w}+4725>0 \Rightarrow \mathrm{m}^2+20 \mathrm{~m}-525<0 \\
& \Rightarrow(\mathrm{m}+35)(\mathrm{m}-15) \leq 0 \Rightarrow \mathrm{m} \text { lies between }-35 \text { and } 15 \text { ie, },-35<\mathrm{m}<15
\end{aligned}
$
Question 4.
The length of the diameter of the circle which touches the $\mathrm{x}$-axis at the point $(1,0)$ and passes through the point $(2,3) \ldots \ldots \ldots$
(a) $\frac{6}{5}$
(b) $\frac{5}{3}$
(c) $\frac{10}{3}$
(d) $\frac{3}{5}$
Solution:
(c) $\frac{10}{3}$
Hint:
Since radius is $\perp \mathrm{r}$ to tangent it passes through the centre $(1, \mathrm{y})$
$\mathrm{AC}=\mathrm{CB}=$ radius
$\mathrm{AC}^2=\mathrm{CB}^2 \Rightarrow \mathrm{y}^2=1+(\mathrm{y}-3)^2$
$6 \mathrm{y}=10 \Rightarrow \mathrm{y}=\frac{5}{3}$
Diameter $2 \mathrm{y}=\frac{10}{3}$
Question 5.
The radius of the circle $3 x^2+b y^2+4 b x-6 b y+b^2=0$ is
(a) 1
(b) 3
(c) $\sqrt{10}$
(d) $\sqrt{11}$
Solution:
(c) $\sqrt{10}$
Hint:
Here $\mathrm{b}=3$
$
\begin{aligned}
& 3 x^2+3 y^2+12 x-18 y+9=0 \\
& (\div 3) z z>x^2+y^2+4 x-6 y+3=0
\end{aligned}
$
Comparing with general form
$
\begin{aligned}
& \mathrm{g}=2, \mathrm{f}=-3, \mathrm{c}=3 \\
& \therefore \text { radius }=\sqrt{g^2+f^2-c}=\sqrt{4+9-3}=\sqrt{10}
\end{aligned}
$
Question 6.
The centre of the circle inscribed in a square formed by the lines $x^2-8 x-12=0$ and $y^2-14 y+45=0$ is
(a) $(4,7)$
(b) $(7,4)$
(c) $(9,4)$
(d) $(4,9)$
Solution:
(a) $(4,7)$
Hint:
$
\begin{aligned}
& (x-6)(x-2)=0 \Rightarrow x=2 \text { or } 6 \\
& (y-9)(y-5)=0 \Rightarrow y=5 \text { or } 9
\end{aligned}
$
$
\begin{aligned}
& P=\text { mid point of } \mathrm{AB}=\left(\frac{2+6}{2}, \frac{5+5}{2}\right)=(4,5) \\
& \mathrm{Q}=\text { mid point of } \mathrm{CD}=\left(\frac{2+6}{2}, \frac{9+9}{2}\right)=(4,9)
\end{aligned}
$
Centre of the circle $=$ Mid point of $\mathrm{PQ}$
$
=\left(\frac{4+4}{2}, \frac{5+9}{2}\right)=(4,7)
$
Question 7.
The equation of the normal to the circle $x^2+y^2-2 x-2 y+1=0$ which is parallel to the line $2 x+4 y=3$ is .
(a) $x+2 y=3$
(b) $x+2 y+3=3$
(c) $2 x+4 y+3=0$
(d) $x-2 y+3=0$
Solution:
(a) $x+2 y=3$
Hint:
Centre of the circle $=(1,1)$ and radius $=r=1$
Normal is parallel to $2 x+4 y=3$
So equation of normal will be the form $2 \mathrm{x}+4 \mathrm{y}=\mathrm{k}$
It passes throug the centre $(1,1) \Rightarrow 2+4=6=\mathrm{k}$
So equation of the normal is $x+4 y=6$
$(\div$ by 2$) x+2 y=3$
Question 8.
If $\mathrm{P}(\mathrm{x}, \mathrm{y})$ be any point on $16 \mathrm{x}^2+25 \mathrm{y}^2=400$ with foci $F_1(3,0)$ and $F_2(-3,0)$ then $P F_1+\mathrm{PF}_2$ is
(a) 8
(b) 6
(c) 10
(d) 12
Solution:
(c) 10
Hint:
$
\mathrm{F}_1 \mathrm{P}+\mathrm{F}_2 \mathrm{P}=2 \mathrm{a}
$
Here the equation is $16 x^2+25 y^2=400$
$
\begin{aligned}
& (\div \mathrm{by} 400) \Rightarrow \frac{16 x^2}{400}+\frac{25 y^2}{400}=1 \Rightarrow \frac{x^2}{25}+\frac{y^2}{16}=1 \\
& \mathrm{a}^2=25 \Rightarrow \mathrm{a}=5 \\
& \therefore 2 \mathrm{a}=10
\end{aligned}
$
Question 9.
The radius of the circle passing through the point $(6,2)$ two of whose diameter are $x+y=6$ and $x+2 y=$ 4 is ..............
(a) 10
(b) $2 \sqrt{5}$
(c) 6
(d) 4
Solution:
(b) $2 \sqrt{5}$
Hint:
The point of intersection of the diameters is the centre.
Now solving $\mathrm{x}+\mathrm{y}=6$.
$
\mathrm{x}+2 \mathrm{y}=4 \ldots \ldots \ldots \ldots \text { (2) }
$
(1) $-(2) \Rightarrow-y=2 \Rightarrow y=-2$
Substituting $y=-6$ in (1)
$
\begin{aligned}
& x-2=6 \Rightarrow x=8 \\
& \text { Centre }=(8,-2) \\
& \text { The circle passes through }(6,2) \\
& \therefore \text { radius }=\sqrt{(8-6)^2+(-2-2)^2}=\sqrt{4+16} \\
& =\sqrt{20}=\sqrt{4 \times 5}=2 \sqrt{5}
\end{aligned}
$
Question 10.
The area of quadrilateral formed with foci of the hyperbolas $\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$ and $\frac{x^2}{a^2}-\frac{y^2}{b^2}=-1$ is
(a) $4\left(a^2+b^2\right)$
(b) $2\left(a^2+b^2\right)$
(c) $\mathrm{a}^2+\mathrm{b}^2$
(d) $\frac{1}{2}\left(\mathrm{a}^2+\mathrm{b}^2\right)$
Solution:
(b) $2\left(a^2+b^2\right)$
Hint:
The foci are $(\pm a e, 0)$ and $(0, \pm a e)$
The above diagram is a rhombus.
Its area $=\frac{1}{2} \mathrm{~d}_1 \mathrm{~d}_2$
$
=\frac{1}{2} \text { (2ae) }(2 \mathrm{ae})=2 \mathrm{a}^2 \mathrm{e}^2
$
But we know $b^2=a^2\left(e^2-1\right)$
$
\begin{aligned}
& \text { i.e., } b^2=a^2 \mathrm{e}^2-\mathrm{a}^2 \\
& \Rightarrow \mathrm{b}^2+\mathrm{a}^2=\mathrm{a}^2 \mathrm{e}^2
\end{aligned}
$
So area $=2\left(a^2+b^2\right)$
Question 11.
If the normals of the parabola $y^2=4 x$ drawn at the end points of its latus rectum are tangents to the circle $(\mathrm{x}-3)^2+(\mathrm{y}+2)^2=\mathrm{r}^2$, then the value of $\mathrm{r}^2$ is
(a) 2
(b) 3
(c) 1
(d) 4
Solution:
(a) 2
Hint:
The normals are $x \pm y=3$
Distance from $(3,-2)$ on both normal is $r$
$
\text { (i.e.) } r=\left|\frac{3-2-3}{\sqrt{2}}\right|=\sqrt{2} \Rightarrow r^2=2
$
Question 12.
If $x+y=k$ is a normal to the parabola $y^2=12 x$, then the value of $k$ is
(a) 3
(b) $-1$
(c) 1
(d) 9
Solution:
(d) 9
Hint:
Slope of normal $=-1$ and Slope of tangent $=-1$
The condition for $\mathrm{y}=\mathrm{mx}+\mathrm{c}$ to be a tangent to the parabola
$
\mathrm{y}^2=4 \mathrm{ax} \text { is } \mathrm{c}=\frac{a}{m}
$
Here the parabola is $y^2=12 x \Rightarrow a=3$ and $\mathrm{m}=1$
$\therefore \mathrm{c}=\frac{3}{1}=3$
$\therefore$ Equation of tangent is $\mathrm{y}=\mathrm{x}+3$
Solving the parabola and tangent we get the points $(3,6)$ and $(3,-6)$ $\therefore \mathrm{k}=-3$ or $\mathrm{k}=9$
Question 13.
The ellipse $\mathrm{E}_1: \frac{x^2}{9}+\frac{y^2}{4}=1$ is inscribed in a rectangle $\mathrm{R}$ whose sides are parallel to the coordinate axes. Another ellipse $E_2$ passing through the point $(0,4)$ circumscribes the rectangle $R$. The eccentricity of the ellipse is ..............
(a) $\frac{\sqrt{2}}{2}$
(b) $\frac{\sqrt{3}}{2}$
(c) $\frac{1}{2}$
(d) $\frac{3}{4}$
Solution:
(c) $\frac{1}{2}$
Hint:
$e=\frac{1}{2}$
Question 14.
Tangents are drawn to the hyperbola $\frac{x^2}{9}-\frac{y^2}{4}=1$ parallel to the straight line $2 x-y=1$. One of the points of contact of tangents on the hyperbola is .............
(a) $\left(\frac{9}{2 \sqrt{2}}, \frac{-1}{\sqrt{2}}\right)$
(b) $\left(\frac{-9}{2 \sqrt{2}}, \frac{1}{\sqrt{2}}\right)$
(c) $\left(\frac{9}{2 \sqrt{2}}, \frac{1}{\sqrt{2}}\right)$
$(d)(3 \sqrt{3},-2 \sqrt{2})$
Solution:
(c) $\left(\frac{9}{2 \sqrt{2}}, \frac{1}{\sqrt{2}}\right)$
Hint:
The tangents are parallel to $2 x-y=1$
So equation of tangents will be of the form $2 \mathrm{x}-\mathrm{y}=\mathrm{k} \Rightarrow \mathrm{y}=2 \mathrm{x}-\mathrm{k}$
Comparing this equation with $\mathrm{y}=\mathrm{mx}+\mathrm{c}$ we get $\mathrm{m}=2$ and $\mathrm{c}=-\mathrm{k}$
The given hyperbola is $\frac{x^2}{9}-\frac{y^2}{4}=1$
$\Rightarrow \mathrm{a}^2=9$ and $\mathrm{b}^2=4$
Now the condition for the line $\mathrm{y}=\mathrm{mx}+\mathrm{c}$ to be a tangent to
$
\begin{aligned}
\frac{x^2}{a^2}-\frac{y^2}{b^2}=1 \text { is } c^2 & =a^2 m^2-b^2 \\
\text { LHS }=c^2=(-k)^2 & =k^2 \\
\text { RHS }=a^2 m^2-b^2 & =9(2)^2-4=32 \\
\text { Now, LHS } & =\text { RHS } \\
k^2=32 \Rightarrow k & =\pm \sqrt{32}=\pm 4 \sqrt{2}
\end{aligned}
$
The point of contact is
$
\begin{aligned}
\left(\frac{-a^2 m}{c}, \frac{-b^2}{c}\right) & =\left(\frac{-a^2 m}{-k}, \frac{-b^2}{-k}\right)=\left(\frac{a^2 m}{k}, \frac{b^2}{k}\right)=\left(\frac{9(2)}{\pm 4 \sqrt{2}}, \frac{4}{\pm 4 \sqrt{2}}\right) \\
& =\left(\frac{9}{2 \sqrt{2}}, \frac{1}{\sqrt{2}}\right)
\end{aligned}
$
Question 15.
The equation of the circle passing through the foci of the ellipse $\frac{x^2}{16}-\frac{y^2}{9}=1$ having centre at $(0,3)$ is
(a) $x^2+y^2-6 y-7=0$
(b) $x^2+y^2-6 y+7=0$
(c) $x^2+y^2-6 y-5=0$
(d) $x^2+y^2-6 y+5=0$
Solution:
(a) $x^2+y^2-6 y-7=0$
The given ellipse is $\frac{x^2}{16}+\frac{y^2}{9}=1$
$
\begin{aligned}
a^2=16 \text { and } b^2 & =9 \\
e^2 & =\frac{a^2-b^2}{a^2}=\frac{16-9}{16}=\frac{7}{16} \Rightarrow e=\frac{\sqrt{7}}{4}
\end{aligned}
$
Now $a=4$ and $e=\frac{\sqrt{7}}{4} \Rightarrow a e=\sqrt{7}$
Now the foci are $(\pm a e, 0)=(\pm \sqrt{7}, 0)$
Centre $=(0,3)$
Passing through the foci $=(\pm \sqrt{7}, 0)$
The distance between centre and foci $=$ radius of the circle
ie., $r=\sqrt{(0 \pm \sqrt{7})^2+(3-0)^2}=\sqrt{7+9}=\sqrt{16}=4$
Now Centre $=(0,3) ; r=4$
Equation of the circle is $(\mathrm{x}-0) 2+(\mathrm{y}-3) 2=42$
ie., $x^2+y^2-6 y+9-16=0$
$x^2+y^2-6 y-7=0$
Question 16.
Let $\mathrm{C}$ be the circle with centre at $(1,1)$ and radius $=1$. If $\mathrm{T}$ is the circle centered at $(0, \mathrm{y})$ passing through the origin and touching the circle $\mathrm{C}$ externally, then the radius of $\mathrm{T}$ is equal
(a) $\frac{\sqrt{3}}{\sqrt{2}}$
(b) $\frac{\sqrt{3}}{2}$
(c) $\frac{1}{2}$
(d) $\frac{1}{4}$
Solution:
(d) $\frac{1}{4}$
Hint:
Equation of circle is $x^2+y^2+2 g x+2 f y+c=0$
i.e., $\left(1+y^2\right)^2=(1-y)^2=1$
Solving we get $y=\frac{1}{4}$
Question 17.
Consider an ellipse whose centre is of the origin and its major axis is along $x$-axis. If its eccentricity is $\frac{3}{5}$ and the distance between its foci is 6 , then the area of the quadrilateral inscribed in the ellipse with diagonals as major and minor axis of the ellipse is
(a) 8
(b) 32
(c) 80
(d) 40
Solution:
(d) 40
Hint:
$
\begin{aligned}
& e=\frac{3}{5} \\
& \begin{aligned}
a e & =3 \\
a^2 & =25
\end{aligned} \Rightarrow a\left(\frac{3}{5}\right)=3 \Rightarrow a=5 \\
&
\end{aligned}
$
$
\text { Now } \begin{aligned}
b^2 & =a^2\left(1-e^2\right) \Rightarrow b^2=25\left(1-\frac{9}{25}\right)=16 \\
a^2 & =25 \Rightarrow a=5 ; 2 a=10 \\
b^2 & =16 \Rightarrow b=4 ; 2 b=8
\end{aligned}
$
Now area of the quadrilateral is $\frac{1}{2}\left(d_1 \times d_2\right)=\frac{1}{2}(10)(8)=40$
Question 18.
Area of the greatest rectangle inscribed in the ellipse $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$ is
(a) $2 \mathrm{ab}$
(b) ab
(c) $\sqrt{a b}$
(d) $\frac{a}{b}$
Solution:
(a) $2 \mathrm{ab}$
Hint:
Area of the greatest rectangle is $(\sqrt{2} a)(\sqrt{2} b)=2 a b$
Question 19.
An ellipse has $O B$ as semi minor axes, $F$ and F' its foci and the angle FBF' is a right angle. Then the eccentricity of the ellipse is ............
(a) $\frac{1}{\sqrt{2}}$
(b) $\frac{1}{2}$
(c) $\frac{1}{4}$
$(d) \frac{1}{\sqrt{3}}$
Solution:
(a) $\frac{1}{\sqrt{2}}$
Hint:
Here $\mathrm{F}_1 \mathrm{~B}+\mathrm{FB}=\mathrm{FF}^{\prime}$
ie., $\left(\sqrt{a^2 e^2+b^2}\right)^2+\left(\sqrt{a^2 e^2+b^2}\right)^2=(2 a e)^2$
$\Rightarrow \quad e=\frac{1}{2}$
Question 20.
The eccentricity of the ellipse $(x-3)^2+(y-4)^2=\frac{y^2}{9}$ is ............
(a) $\frac{\sqrt{3}}{2}$
(b) $\frac{1}{3}$
(c) $\frac{1}{3 \sqrt{2}}$
(d) $\frac{1}{\sqrt{3}}$
Solution:
(b) $\frac{1}{3}$
Hint:
The given equation is of the form $\mathrm{FP}^2=\mathrm{e}^2 \mathrm{PM}^2$
$
\begin{aligned}
& \text { i.e., }(\mathrm{x}-3)^2+(\mathrm{y}-4)^2=\frac{1}{9}\left(\mathrm{y}^2\right) \\
& \Rightarrow \mathrm{e}^2=\frac{1}{9} \Rightarrow \mathrm{e}=\frac{1}{3}
\end{aligned}
$
Question 21.
If the two tangents drawn from a point $P$ to the parabola $y^2=4 x$ are at right angles then the locus of $P$ is
(a) $2 x+1=0$
(b) $x=-1$
(c) $2 x-1=0$
(d) $x=1$
Solution:
(b) $x=-1$
Hint:
When the tangents at drawns from a point on the directrix are at right angles.
So equation of directrix to $y^2=4 x$ is $x=-1$
Question 22.
The circle passing through $(1,-2)$ and touching the axis of $x$ at $(3,0)$ passing through the point
(a) $(-5,2)$
(b) $(2,-5)$
(c) $(5,-2)$
(d) $(-2,5)$
Solution:
(c) $(5,-2)$
Hint:
Let the centre be $(3, \mathrm{~h})$ $\mathrm{r}=\mathrm{k}$
Equation of the circle is
$
(\mathrm{x}-3)^2+(\mathrm{y}-\mathrm{h})^2=\mathrm{r}^2=(\mathrm{k})^2
$
It passes through $(1,-2) \Rightarrow \mathrm{k}^2=8$
Substituting $(5,-2)$ the equation satisfies.
Question 23.
The locus of a point whose distance from $(-2,0)$ is $\frac{2}{3}$ times its distance from the line $x=\frac{-9}{2}$ is
(a) a parabola
(b) a hyperbola
(c) an ellipse
(d) a circle
Solution:
(c) an ellipse
Hint:
Here $\mathrm{e}=\frac{2}{3}<1$
So the conic is an ellipse
Question 24.
The values of $m$ for which the line $y=m x+2 \sqrt{5}$ touches the hyperbola $16 x^2-9 y^2=144$ are the roots of $x^2-(a+b) x-4=0$, then the value of $(a+b)$ is
(a) 2
(b) 4
(c) 0
(d) $-2$
Solution:
(c) 0
Hint:
$
\begin{aligned}
& 16 x^2-9 y^2=144 \Rightarrow \frac{16 x^2}{144}-\frac{9 y^2}{144}=1 \\
& \text { ie., } \frac{x^2}{9}-\frac{y^2}{16}=1
\end{aligned}
$
Here $a^2=9 ; b^2=16$
The condition for the line $y=m x+c$ to be a tangent to $\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$ is
$
c^2=a^2 m^2-b^2
$
Here the given line is $\mathrm{y}=\mathrm{mx}+2 \sqrt{5}$
$\Rightarrow \mathrm{m}=\mathrm{m}$ and $\mathrm{c}=2 \sqrt{5}$
The condition is $\mathrm{c}^2=\mathrm{a}^2 \mathrm{~m}^2-\mathrm{b}^2$
ie., $(2 \sqrt{5})^2=9\left(\mathrm{~m}^2\right)-16$
$\Rightarrow 9 \mathrm{~m}^2=20+16=36 \Rightarrow \mathrm{m}^2=4$
i.e., $\mathrm{m}=\pm 2$
$\mathrm{a}=2, \mathrm{~b}=-2$ (say)
So $a+b=2-2=0$
Question 25.
If the coordinates at one end of a diameter of the circle $x^2+y^2-8 x-4 y+c=0$ are $(11,2)$, the coordinates of the other end are
(a) $(-5,2)$
(b) $(-3,2)$
(c) $(5,-2)$
(d) $(-2,5)$
Solution:
(b) $(-3,2)$
Hint:
Centre $=(4,2)$ one end of diameter $(11,2) \quad(11,2) \quad(4,2) \quad(x, y)$
Let the other end be $(x, y)$
Now centre $=$ mid point of the end points of diameter.
$\Rightarrow(4,2)=\left(\frac{11+x}{2}, \frac{2+y}{2}\right)$
$\begin{aligned}
& \Rightarrow \begin{array}{l|l}
\Rightarrow \frac{11+x}{2}=4 & \frac{2+y}{2}=2
\end{array} \\
& \begin{array}{l|l}
11+x=8 & 2+y=4
\end{array} \\
& \begin{array}{l|l}
x=-3 & y=4-2=2
\end{array} \\
&
\end{aligned}$