Let the common tangents to the curves $4({x^2} + {y^2}) = 9$ and ${y^2} = 4x$ intersect at the point Q. Let an ellipse, centered at the origin O, has lengths of semi-minor and semi-major axes equal to OQ and 6, respectively. If e and l respectively denote the eccentricity and the length of the latus rectum of this ellipse, then ${l \over {{e^2}}}$ is equal to ______________.
Answer (integer)
4
Solution
<p>Let y = mx + c is the common tangent</p>
<p>So $$c = {1 \over m} = \pm \,{3 \over 2}\sqrt {1 + {m^2}} \Rightarrow {m^2} = {1 \over 3}$$</p>
<p>So equation of common tangents will be $y = \pm \,{1 \over {\sqrt 3 }}x \pm \,\sqrt 3$, which intersects at Q($-$3, 0)</p>
<p>Major axis and minor axis of ellipse are 12 and 6.</p>
<p>So eccentricity</p>
<p>${e^2} = 1 - {1 \over 4} = {3 \over 4}$ and length of latus rectum $= {{2{b^2}} \over a} = 3$</p>
<p>Hence, ${l \over {{e^2}}} = {3 \over {3/4}} = 4$</p>
About this question
Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola
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