If S and S′ are the foci of the ellipse x218 + y29 = 1 and P be a point on the ellipse, then (SP S'P) + (SP S'P) is equal to :
- 3(6 + 2)
- 3(1 + 2)
- 27
- 9
Answer
(3) 27
Solution
a2 = 18, b2 = 9 e2 = 1 - b2a2 = 12
For P(x, y) on the ellipse (focal distances): SP = a - ex, S'P = a + ex
SP S'P = a2 - e2x2 = 18 - x22, where 0 x2 18
Maximum (at x = 0): 18
Minimum (at x2 = 18): 18 - 9 = 9
+ = 9 + 18 = 27
Answer: (3)