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JEE Main 20178 AprilOnline
Exam
JEE Main
Year
2017
Date
8 April
Mode
Online
Subject
Mathematics
Chapter
Straight Lines

JEE Main 2017 · 8 April · Online · Mathematics · Straight Lines

The locus of the point of intersection of the straight lines, tx -...

The locus of the point of intersection of the straight lines, tx−2y−3t=0tx - 2y - 3t = 0 and x−2ty+3=0(t∈R),x - 2ty + 3 = 0\left(t \in R\right), is:

Options

  1. AA hyperbola with the length of conjugate axis 3
  2. BA hyperbola with eccentricity 5\sqrt{5}
  3. CAn ellipse with the length of major axis 6
  4. DAn ellipse with eccentricity 25\dfrac{2}{\sqrt{5}}

Correct answer

A. A hyperbola with the length of conjugate axis 3

Step-by-step solution

tx - 2y - 3t = 0 . . . . . . . (1)<br/><br/>\left(1\right)<br /><br />x - 2ty + 3 = 0 . . . . . . \left(2\right),</p><p>Multiplyby,</p><p>Multiply bytandsubtractfromaboveequation,weget</p><p>and subtract from above equation, we get</p><p>y\left(2{t}^{2} - 2\right) = 6t</p><p>Nowmultiplyby</p><p>Now multiply bytinfirstequationandsubtractsecond,weget</p><p>(t2−1)x=3(t2−1)in first equation and subtract second, we get</p><p>\left({t}^{2} - 1\right)x = 3 \left({t}^{2} - 1\right)

⇒x=3(t2+1)t2−1⇒ x = 3\dfrac{{( t}^{2} + 1 )}{{t}^{2} - 1}

⇒x29=(t2+1t2−1)2 and  2y3=2tt2−1⇒ \dfrac{{x}^{2}}{9} = {\left(\dfrac{{t}^{2} + 1}{{t}^{2} - 1}\right)}^{2} \text{\text{ and } } \dfrac{2y}{3} = \dfrac{2t}{{t}^{2} - 1}

⇒4y29=4t2(t2−1)2⇒ \dfrac{4{y}^{2}}{9} = \dfrac{4{t}^{2}}{{\left({t}^{2} - 1\right)}^{2}}

⇒x29−4y29=1⇒ \dfrac{{x}^{2}}{9} - \dfrac{4{y}^{2}}{9} = 1

⇒x29−y294=1⇒ \dfrac{{x}^{2}}{9} - \dfrac{{y}^{2}}{\dfrac{9}{4}} = 1

a2=9;⇒a=3{a}^{2} = 9; ⇒ a=3

b2=94⇒b=32{b}^{2} = \dfrac{9}{4}⇒ b = \dfrac{3}{2}
∴∴ Length of conjugate axis =2b=2×32=3= 2b = 2 \times \dfrac{3}{2} = 3

Length of transverse axis=2a=6= 2a = 6

e2=1+949=1+14;e=52{e}^{2} = 1 + \dfrac{\dfrac{9}{4}}{9} = 1 + \dfrac{1}{4}; e = \dfrac{\sqrt{5}}{2}

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