Let the domain of the function f(x) = 3 5 7 (9x - x2 - 13 ) be the interval (m, n). Let the hyperbola x2a2 - y2b2 = 1 have eccentricity n3 and the length of the latus rectum 8m3. Then b2 - a2 is equal to :
- 7
- 9
- 11
- 5
Answer
(1) 7
Solution
For f(x) to be defined: 5 7 (9x - x2 - 13) > 0
7 (9x - x2 - 13) > 1
9x - x2 - 13 > 7
x2 - 9x + 20 < 0
(x - 4)(x - 5) < 0 x (4, 5)
m = 4, n = 5
Eccentricity e = 53 1 + b2a2 = 259 b2 = 16a29
Latus rectum 2b2a = 323 b2 = 16a3
16a29 = 16a3 a = 3
a2 = 9, b2 = 16
b2 - a2 = 16 - 9 = 7
Answer: (1)