Let one focus of the hyperbola H : x2a2 - y2b2 = 1 be at ( 10, 0) and the corresponding directrix be x = 9 10. If e and l respectively are the eccentricity and the length of the latus rectum of H, then 9(e2 + l) is equal to :
- 12
- 14
- 15
- 16
Answer
(4) 16
Solution
Focus: ae = 10; directrix: ae = 9 10
Multiplying: a2 = 9 a = 3
e = 103 e2 = 109
b2 = a2(e2 - 1) = 9 19 = 1
l = 2b2a = 23
9(e2 + l) = 9 ( 109 + 23 ) = 10 + 6 = 16
Answer: (4)