The line $x=8$ is the directrix of the ellipse $\mathrm{E}:\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ with the corresponding focus $(2,0)$. If the tangent to $\mathrm{E}$ at the point $\mathrm{P}$ in the first quadrant passes through the point $(0,4\sqrt3)$ and intersects the $x$-axis at $\mathrm{Q}$, then $(3\mathrm{PQ})^{2}$ is equal to ____________.
Answer (integer)
39
Solution
$\begin{aligned} & \frac{a}{e}=8 \\\\ & a e=2 .(1) \\\\ & 8 e=\frac{2}{e} \\\\ & e^2=\frac{1}{4} \Rightarrow e=\frac{1}{2} \\\\ & a=4 \\\\ & b^2=a^2\left(1-e^2\right) \\\\ & =16\left(\frac{3}{4}\right)=12 \\\\ & \frac{x \cos \theta}{4}+\frac{y \sin \theta}{2 \sqrt{3}}=1 \\\\ & \sin \theta=\frac{1}{2} \\\\ & \theta=30^{\circ}\end{aligned}$
<br/><br/>$\begin{aligned} & \mathrm{P}(2 \sqrt{3}, \sqrt{3}) \\\\ & \mathrm{Q}\left(\frac{8}{\sqrt{3}}, 0\right) \\\\ & (3 \mathrm{PQ})^2=39\end{aligned}$
About this question
Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola
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