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JEE Advanced 2021Paper 1
Exam
JEE Advanced
Year
2021
Paper
Paper 1
Subject
Mathematics
Chapter
Straight Lines

JEE Advanced 2021 · Paper 1 · Mathematics · Straight Lines

Consider the lines L_ 1 and L_ 2 defined by L_ 1 : x 2 + y-1=0 and...

Consider the lines L1L_{1} and L2L_{2} defined by
\\ L_{1}: x 2+\sqrt{2}+y-1=0  and \text{ and }L_{2}: x 2−\sqrt{2}-y+1=0 \\ \\ For a fixed constant λ\lambda, let CC be the locus of a point PP such that the product of the distance of PP from L1L_{1} and the distance of PP from L2L_{2} is λ2\lambda^{2}. The line y=2x+1y=2 x+1 meets CC at two points RR and SS, where the distance between RR and SS is 270\sqrt{270}. \\ Let the perpendicular bisector of RSR S meet CC at two distinct points R′R^{\prime} and S′S^{\prime}. Let DD be the square of the distance between R′R^{\prime} and S′S^{\prime}.

The value of DD is

Correct answer

0

Step-by-step solution

From the first question

The equation of the locus is ∣2x2−(y−1)2∣=27\left|2{x}^{2} - ( y - 1{)}^{2}\right| = 27

The line is y=2x+1y = 2x + 1 or y−1=2xy - 1 = 2x

By substituting the value of yy in the equation of the curve CC, we get

\left|2{x}^{2} - ( y - 1{)}^{2}\right| = 27</p><p></p><p>⇒ \left|2{x}^{2} - ( 2x{)}^{2}\right| = 27</p><p></p><p>⇒ 2{x}^{2} = 27</p><p></p><p>⇒ x = \pm 3\sqrt{\dfrac{3}{2}}</p><p></p><p>⇒ {x}_{1}, {x}_{2} = \pm 3\sqrt{\dfrac{3}{2}}</p><p>Let</p><p>LetMbethemid−pointofbe the mid-point of{R}^{'} \text{ and } {S}^{'}</p><p>So,the</p><p>So, thexcoordinateofcoordinate ofTisis\dfrac{{x}_{1} + {x}_{2}}{2} = 0</p><p>Itlieson</p><p>It lies ony = 2x + 1</p><p>So,thecoordinatesof</p><p>So, the coordinates ofTare(0,1)are \left(0, 1\right)

Slope of the line perpendicular to y=2x+1y = 2x + 1 is −12\dfrac{- 1}{2}

So, the equation of perpendicular bisector is

y−1=−12(x−0)y - 1 = \dfrac{- 1}{2}\left(x - 0\right)

Or, x+2y=2x + 2y = 2

Let coordinate of R′ and S′are(p1,q1) and (p2,q2){R}^{'} \text{ and } {S}^{'} are \left({p}_{1}, {q}_{1}\right) \text{ and } \left({p}_{2}, {q}_{2}\right), we get

D=(p2−p1)2+(q2−q1)2D = {\left({p}_{2} - {p}_{1}\right)}^{2} + {\left({q}_{2} - {q}_{1}\right)}^{2}

Since both the points satisfy the equation of the line, we get

D=[2(q2−q1)]2+(q2−q1)2D = {\left[2\left({q}_{2} - {q}_{1}\right)\right]}^{2} + {\left({q}_{2} - {q}_{1}\right)}^{2}

D=5(q2−q1)2D = 5{\left({q}_{2} - {q}_{1}\right)}^{2}

Solving, x+2y=2x + 2y = 2 with 2x2−(y−1)2=272{x}^{2} - ( y - 1{)}^{2} = 27, we get

2(2−2y)2−(y−1)2=272{\left(2 - 2y\right)}^{2} - ( y - 1{)}^{2} = 27

⇒7(y−1)2=27⇒ 7 ( y - 1{)}^{2} = 27

⇒y−1=±337⇒ y - 1 = \pm 3\sqrt{\dfrac{3}{7}}

⇒y=1±337⇒ y = 1 \pm 3\sqrt{\dfrac{3}{7}}

⇒q1,q2=1±337⇒ {q}_{1}, {q}_{2} = 1 \pm 3\sqrt{\dfrac{3}{7}}

So, (q2−q1)2=(637)2{\left({q}_{2} - {q}_{1}\right)}^{2} = {\left(6\sqrt{\dfrac{3}{7}}\right)}^{2}

Hence, D=5(q2−q1)2=5×(637)2=77.14D = 5{\left({q}_{2} - {q}_{1}\right)}^{2} = 5 \times {\left(6\sqrt{\dfrac{3}{7}}\right)}^{2} = 77 . 14

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