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JEE Main 202125 JulyMorning Shift
Exam
JEE Main
Year
2021
Date
25 July
Shift
Morning Shift
Subject
Mathematics
Chapter
Ellipse

JEE Main 2021 · 25 July · Morning Shift · Mathematics · Ellipse

Let an ellipse E: x ^ 2 a ^ 2 + y ^ 2 b ^ 2 = 1, a ^ 2 > b ^ 2 ,...

Let an ellipse E:x2a2+y2b2=1,a2>b2E: \dfrac{{x}^{2}}{{a}^{2}} + \dfrac{{y}^{2}}{{b}^{2}} = 1, {a}^{2} > {b}^{2}, passes through (32,1)\left(\sqrt{\dfrac{3}{2}}, 1\right) and has eccentricity 13\dfrac{1}{\sqrt{3}}. If a circle, centered at focus F(α,0),α>0F ( \alpha, 0 ) , \alpha > 0, of EE and radius 23\dfrac{2}{\sqrt{3}}, intersects EE at two points PandQP and Q, then PQ2P{Q}^{2} is equal to :

Options

  1. A83\dfrac{8}{3}
  2. B43\dfrac{4}{3}
  3. C163\dfrac{16}{3}
  4. D33

Correct answer

C. 163\dfrac{16}{3}

Step-by-step solution

The ellipse x2a2+y2b2=1\dfrac{{x}^{2}}{{a}^{2}} + \dfrac{{y}^{2}}{{b}^{2}} = 1 passes through the point (32,1)\left(\sqrt{\dfrac{3}{2}}, 1\right)

So, 32a2+1b2=1...(1)\dfrac{3}{2{a}^{2}} + \dfrac{1}{{b}^{2}} = 1 . . . \left(1\right) and 1−b2a2=13...1 - \dfrac{{b}^{2}}{{a}^{2}} = \dfrac{1}{3} . . .\left(2\right)</p><p>Onsolvingequations(1)</p><p>On solving equations \left(1\right) and \left(2\right),weget</p><p>, we get</p><p>⇒ {a}^{2} = 3 \text{ and } {b}^{2} = 2</p><p></p><p>⇒ \dfrac{{x}^{2}}{3} + \dfrac{{y}^{2}}{2} = 1 . . . \left(3\right)</p><p>Weknowthat,fociioftheellipse</p><p>We know that, focii of the ellipse\dfrac{{x}^{2}}{{a}^{2}} + \dfrac{{y}^{2}}{{b}^{2}} = 1is(±ae,0)is \left(\pm ae, 0\right)

Here, a=3 and e=13a = \sqrt{3} \text{ and } e = \dfrac{1}{\sqrt{3}}

∴∴ Its focus is \left(1, 0\right)\left(∵ \alpha > 0\right)</p><p>Now,equationofcircleis</p><p></p><p>Now, equation of circle is</p><p>( x - 1{)}^{2} + {y}^{2} = \dfrac{4}{3} . . . \left(4\right)</p><p>Solving(3)</p><p>Solving \left(3\right) and (4)\left(4\right) we get

y=±23,x=1y = \pm \dfrac{2}{\sqrt{3}}, x = 1

⇒PQ2=(1−1)2+(23+23)2⇒ P{Q}^{2} = \sqrt{{\left(1 - 1\right)}^{2} + {\left(\dfrac{2}{\sqrt{3}} + \dfrac{2}{\sqrt{3}}\right)}^{2}}

=(43)2=163= {\left(\dfrac{4}{\sqrt{3}}\right)}^{2} = \dfrac{16}{3}

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