Mathematics · Ellipse

JEE Main 2025 — 29 January, Morning Shift — Question 56

Let the ellipses E1: x2a2+y2b2=1,a>bE_1:\ \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1,\quad a > b and E2: x2A2+y2B2=1,A<BE_2:\ \frac{x^2}{A^2} + \frac{y^2}{B^2} = 1,\quad A < B have the same eccentricity 13\dfrac{1}{\sqrt{3}}. Let the product of their lengths of latus rectum be 323,\frac{32}{\sqrt{3}}, and the distance between the foci of E1E_1 be 44. If E1E_1 and E2E_2 meet at points A,B,C,A, B, C, and DD, then the area of the quadrilateral ABCDABCD equals:

  1. Option A:

    666 \sqrt{6}

  2. Option B:

    1865\frac{18 \sqrt{6}}{5}

  3. Option C:

    1265\frac{12 \sqrt{6}}{5}

  4. Option D:

    2465\frac{24 \sqrt{6}}{5}

    Correct

Answer: D

Step-by-step solution

2ae=4\quad 2 \mathrm{ae}=4

2a(13)=42 \mathrm{a}\left(\frac{1}{\sqrt{3}}\right)=4

⇒a=23\Rightarrow \mathrm{a}=2 \sqrt{3}

⇒1−b212=13\Rightarrow 1-\frac{\mathrm{b}^{2}}{12}=\frac{1}{3}

⇒ b2=8\Rightarrow \mathrm{~b}^{2}=8

Now 2 b2a⋅2 A2 B=323\frac{2 \mathrm{~b}^{2}}{\mathrm{a}} \cdot \frac{2 \mathrm{~A}^{2}}{\mathrm{~B}}=\frac{32}{\sqrt{3}}

⇒2(823)2 A2 B=323\Rightarrow 2\left(\frac{8}{2 \sqrt{3}}\right) \frac{2 \mathrm{~A}^{2}}{\mathrm{~B}}=\frac{32}{\sqrt{3}}

⇒A2=2B\Rightarrow A^{2}=2 B

1−A2 B2=131-\frac{\mathrm{A}^{2}}{\mathrm{~B}^{2}}=\frac{1}{3}

⇒1−2 B B2=13 \Rightarrow 1-\frac{2 \mathrm{~B}}{\mathrm{~B}^{2}}=\frac{1}{3}

⇒ B=3 \Rightarrow \mathrm{~B}=3 ⇒A2=6\Rightarrow A^{2}=6

x212+y28=1\frac{x^{2}}{12}+\frac{y^{2}}{8}=1

x26+y29=1\frac{x^{2}}{6}+\frac{y^{2}}{9}=1

On solving (1) & (2) we get

(x,y)≡(65,65),(−65,65),(65,−65),(−65,−65)(x, y) \equiv\left(\frac{\sqrt{6}}{\sqrt{5}}, \frac{6}{\sqrt{5}}\right),\left(\frac{-\sqrt{6}}{\sqrt{5}}, \frac{6}{\sqrt{5}}\right),\left(\frac{\sqrt{6}}{\sqrt{5}}, \frac{-6}{\sqrt{5}}\right),\left(\frac{-\sqrt{6}}{\sqrt{5}}, \frac{-6}{\sqrt{5}}\right)

The four points are vertices of rectangle and its area == 2465\frac{24 \sqrt{6}}{5}

Answer key and solution verified before publishing.

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Exam
JEE Main 2025
Subject
Mathematics
Chapter
Ellipse
Topic
Special properties of ellipse