Let e1 and e2 be the eccentricities of the ellipse x2b2 + y225 = 1 and the hyperbola x216 - y2b2 = 1, respectively. If b < 5 and e1 e2 = 1, then the eccentricity of the ellipse having its axes along the coordinate axes and passing through all four foci (two of the ellipse and two of the hyperbola) is :
- 45
- 35
- 74
- 32
Answer
(2) 35
Solution
Ellipse (b < 5, major axis along y-axis): e12 = 1 - b225
Hyperbola: e22 = 1 + b216
e12 e22 = 1 (25 - b2)(16 + b2)400 = 1
400 + 9b2 - b4 = 400 b2 = 9
Foci of ellipse: (0, 5e1) = (0, 4)
Foci of hyperbola: ( 4e2, 0) = ( 5, 0)
Required ellipse through ( 5, 0) and (0, 4): x225 + y216 = 1
e = 1 - 1625 = 35
Answer: (2)