Medium INTEGER +4 / -1 PYQ · JEE Mains 2024

Let a conic $C$ pass through the point $(4,-2)$ and $P(x, y), x \geq 3$, be any point on $C$. Let the slope of the line touching the conic $C$ only at a single point $P$ be half the slope of the line joining the points $P$ and $(3,-5)$. If the focal distance of the point $(7,1)$ on $C$ is $d$, then $12 d$ equals ________.

Answer (integer) 75

Solution

<p>As per given condition</p> <p>$$\begin{gathered} \frac{d y}{d x}=\frac{y+5}{2(x-3)} \\ \Rightarrow \ln (y+5)=\frac{1}{2} \ln (x-3)+c \\ \text { Passes through }(4,-2) \Rightarrow \ln 3=\frac{1}{2} \ln 1+c \\ \Rightarrow c=\ln 3 \end{gathered}$$</p> <p>$\Rightarrow$ Curve is $(y+5)^2=9(x-3)$</p> <p>Focal distance of $(7,1)=\frac{9}{4}+4=\frac{25}{4}=d$</p> <p>$12 d=75$</p>

About this question

Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola

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