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Q.
List I List II
P If the distance between two parallel tangents having slope $m$ drawn to the hyperbola $\frac{x^2}{9}-\frac{y^2}{49}=1$ is 2 , then $\frac{2}{5}| m |$ is equal to 1 5
Q If a variable line has its intercepts on the coordinate axes e and e', where $\frac{ e }{2}$ and $\frac{ e ^{\prime}}{2}$ are the eccentricities of a hyperbola and its conjugate hyperbola, then the line always touches the circle $x^2+y^2=r^2$, where $r$ is equal to 2 4
R If the equation of line touching both parabolas $y^2-4 x=0$ and $x ^2+32 y =0$ is $ax -2 y + b =0( a , b \in R )$, then $( a + b )$ is equal to 3 2
S If the mid-point of a chord of the ellipse $\frac{x^2}{16}+\frac{y^2}{25}=1$ is $M(0,3)$ and the length of chord is $\frac{8 p }{5}$, then $p$ is equal to 4 1

Conic Sections

Solution:

(P) The equation of tangents to the hyperbola having slope $m$ are $y = mx \pm \sqrt{9 m ^2-49}$.
Now, $\frac{2 \sqrt{9 m ^2-49}}{\sqrt{1+ m ^2}}=2$ (Given)
$\Rightarrow m = \pm \frac{5}{2} \Rightarrow \frac{2}{5}| m |=1 . $
(Q) $A s, \frac{e}{2}$ and $\frac{ e ^{\prime}}{2}$ are eccentricities of a hyperbola and its conjugate hyperbola, so
$\frac{4}{ e ^2}+\frac{4}{ e ^{\prime 2}}=1 \Rightarrow 4=\frac{ e ^2 e ^{\prime 2}}{ e ^2+ e ^{\prime 2}}$ ....(1)The equation of line is
$e^{\prime} x+e y-e e^{\prime}=0$
It is tangent to circle $x^2+y^2=r^2$
$\therefore \frac{e^{\prime}}{\sqrt{ e ^2+ e ^{\prime 2}}}= r =2 \text { [using(1)] }$
(R) The equation of common tangent is $x-2 y+4=0$
$\therefore (a+b)=5$
(S) The equation of chord to ellipse whose midpoint is $(0,3)$ is $\left(T= S _1\right)$
$\Rightarrow y =3$. It intersects the ellipse
$\frac{x^2}{16}+\frac{y^2}{25}=1 \Rightarrow \frac{x^2}{16}=1-\frac{9}{25} \Rightarrow x= \pm \frac{16}{5} \text {. }$
$\therefore$ Length of chord $=\frac{32}{5} \Rightarrow p=4$