Q37 / 91 · KVPY 2018 (SB/SX) · Mathematics

Let x2a2+y2b2=1,a>b be an ellipse with foci F1 and F2 Let AO be its semi-minor axis, where O is the centre of the ellipse. The lines AF1 and AF2, when extended, cut the ellipse again at points B and C respectively. Suppose that the ΔABC is equilateral. Then, the eccentricity of the ellipse is

  • A
    12
  • B
    13
  • C
    13
  • Correct answer
    D
    12

Solution

Given, x2a2+y2b2=1

ABC is an equilateral traingles.

Figure

In ΔAOF2,

tan30°=OF2OA

13=aeb

⇒b=3ae

⇒b2=3a2e2

⇒b2a2=3e2

⇒1-e2=3e2  ∵e=1-b2a2

⇒4e2=1

⇒e2=14⇒e=12

Hence, the correct answer is option: 'D'.

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