Consider a hyperbola H having centre at the origin and foci on the x-axis. Let C1 be the circle touching the hyperbola H and having the centre at the origin. Let C2 be the circle touching the hyperbola H at its vertex and having the centre at one of its foci. If areas (in sq units) of C1 and C2 are 36 and 4 , respectively, then the length (in units) of latus rectum of H is
- 283
- 113
- 143
- 103
Answer
(1) 283
Solution
H: x2a2 - y2b2 = 1, vertex A(a, 0), focus S(ae, 0)
[figure]
∵ C1 (centre O) touches H at its vertices ⇒ radius = a ⇒ a2 = 36 ⇒ a = 6
∵ C2 (centre S) touches H at the nearer vertex A ⇒ radius = SA = ae - a
(ae - a)2 = 4 6(e - 1) = 2 e = 43
b2 = a2(e2 - 1) = 36 79 = 28
Latus rectum = 2b2a = 566 = 283
Answer: (1)