Let the length of the latus rectum of an ellipse x2a2 + y2b2 = 1, (a > b), be 30. If its eccentricity is the maximum value of the function f(t) = - 34 + 2t - t2, then (a2 + b2) is equal to
- 276
- 516
- 256
- 496
Answer
(4) 496
Solution
f(t) = - 34 + 2t - t2 = 14 - (t - 1)2
maximum value = 14 (at t = 1)
e = 14
Length of latus rectum = 2b2a = 30
b2 = 15a
Also b2 = a2(1 - e2) = a2 (1 - 116 ) = 15a216
15a = 15a216
a = 16
a2 = 256 and b2 = 15 16 = 240
a2 + b2 = 256 + 240 = 496
Answer: (4)