The eccentricity of an ellipse E with centre at the origin O is 32 and its directrices are x= 4 63. Let H: x2a2- y2b2=1 be a hyperbola whose eccentricity is equal to the length of semi-major axis of E, and whose length of latus rectum is equal to the length of minor axis of E. Then the distance between the foci of H is :
- 4 2 7
- 4 27
- 4 7
- 87
Answer
(4) 87
Solution
Directrices x= constant, so the major axis of E is along the x-axis; e= 32
Ae= 4 63 A= 4 63 32=2 2 (semi-major axis)
B2=A2 (1-e2 )=8 14=2 minor axis 2B=2 2
Hyperbola H: eH=A=2 2
1+ b2a2=8 b2=7a2
Latus rectum of H = 2b2a=2 2
14a2a=2 2 a= 27
Distance between foci =2aeH=2 27 2 2= 87
Answer: (4)