Let f(x) = x2 + 9, g(x) = xx - 9 and a = f g(10), b = g f(3). If e and l denote the eccentricity and the length of the latus rectum of the ellipse x2a + y2b = 1, then 8e2 + l2 is equal to.
- 6
- 12
- 8
- 16
Answer
(3) 8
Solution
g(10) = 101 = 10 a = f(10) = 109; f(3) = 18 b = g(18) = 189 = 2
Ellipse x2109 + y22 = 1 has its major axis along the x-axis (since 109 > 2)
e2 = 1 - 2109 = 107109 and l = 2 2 109 l2 = 16109
8e2 + l2 = 856 + 16109 = 872109 = 8
Answer: (3)