Let the sum of the focal distances of the point P(4, 3) on the hyperbola H: x2a2 - y2b2 = 1 be 8 53. If for H, the length of the latus rectum is l and the product of the focal distances of the point P is m, then 9l2 + 6m is equal to :
- 187
- 184
- 186
- 185
Answer
(4) 185
Solution
For a point (x1, y1) on the right branch, focal distances are ex1 - a and ex1 + a.
Sum = 2ex1 = 8e = 8 53 e2 = 53
b2 = a2(e2 - 1) = 2a23
P lies on H: 16a2 - 9b2 = 1 16a2 - 272a2 = 1
52a2 = 1 a2 = 52, b2 = 53
l = 2b2a l2 = 4b4a2 = 4 259 52 = 409
9l2 = 40
m = (ex1 - a)(ex1 + a) = e2x12 - a2
= 53 16 - 52 = 1456
6m = 145
9l2 + 6m = 40 + 145 = 185
Answer: (4)