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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