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JEE Main 20211 SeptemberEvening Shift
Exam
JEE Main
Year
2021
Date
1 September
Shift
Evening Shift
Subject
Mathematics
Chapter
Ellipse

JEE Main 2021 · 1 September · Evening Shift · Mathematics · Ellipse

Let θ be the acute angle between the tangents to the ellipse x ^ 2...

Let θ\theta be the acute angle between the tangents to the ellipse x29+y21=1\dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{1} = 1 and the circle x2+y2=3{x}^{2} + {y}^{2} = 3 at their point of intersection in the first quadrant. Then tanθtan\theta is equal to :

Options

  1. A523\dfrac{5}{2\sqrt{3}}
  2. B43\dfrac{4}{\sqrt{3}}
  3. C23\dfrac{2}{\sqrt{3}}
  4. D22

Correct answer

C. 23\dfrac{2}{\sqrt{3}}

Step-by-step solution

Given, x29+y21=1...(1)\dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{1} = 1 . . . \left(1\right)

x2+y2=3...(2){x}^{2} + {y}^{2} = 3 . . . \left(2\right)

On solving equations \left(1\right) \text{ and } \left(2\right),weget</p><p>, we get</p><p>\dfrac{{x}^{2}}{9} + \dfrac{3 - {x}^{2}}{1} = 1</p><p></p><p>⇒ {x}^{2} + 27 - 9{x}^{2} = 9</p><p></p><p>⇒ 8{x}^{2} = 18</p><p></p><p>⇒ {x}^{2} = \dfrac{9}{4}$⇒x=±32⇒ x = \pm \dfrac{3}{2}

⇒x=32⇒ x = \dfrac{3}{2}

Since, point of intersection in the first quadrant.

Put, x=32x = \dfrac{3}{2} in equation \left(2\right),weget, we gety = \dfrac{\sqrt{3}}{2}.</p><p>Hence,thepointofintersectionofthecurves. </p><p>Hence, the point of intersection of the curves\dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{1} = 1andand{x}^{2} + {y}^{2} = 3inthefirstquadrantis(32,32)in the first quadrant is \left(\dfrac{3}{2}, \dfrac{\sqrt{3}}{2}\right)

Now, differentiate the equation \left(1\right)onbothsideswithrespecttoon both sides with respect tox,weget</p><p>, we get</p><p>{y}^{'} = \dfrac{- x}{9y}</p><p></p><p>∴SlopeoftangenttotheellipseSlope of tangent to the ellipse\dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{1} = 1atat\left(\dfrac{3}{2}, \dfrac{\sqrt{3}}{2}\right)isis{m}_{1} = - \dfrac{1}{3\sqrt{3}}</p><p>Now,differentiatetheequation</p><p>Now, differentiate the equation\left(2\right)onbothsideswithrespecttoon both sides with respect tox,weget</p><p>, we get</p><p>{y}^{'} = \dfrac{- x}{y}</p><p></p><p>∴SlopeoftangenttothecircleSlope of tangent to the circle{x}^{2} + {y}^{2} = 3atat\left(\dfrac{3}{2}, \dfrac{\sqrt{3}}{2}\right)isis{m}_{2} = - \sqrt{3}</p><p>So,ifacuteanglebetweenbothcurvesis</p><p>So, if acute angle between both curves is\thetathenthentan\theta = \left|\dfrac{{m}_{1} - {m}_{2}}{1 + {m}_{1}{m}_{2}}\right| = \left|\dfrac{- \dfrac{1}{3\sqrt{3}} + \sqrt{3}}{1 + \left(- \dfrac{1}{3\sqrt{3}}\right) ( - \sqrt{3} )}\right| = \left(\dfrac{2}{\sqrt{3}}\right)$

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