Let P be a parabola with vertex (2,3) and directrix 2x+y=6. Let an ellipse E: x2a2+ y2b2=1, a>b, of eccentricity 1 2 pass through the focus of the parabola P. Then, the square of the length of the latus rectum of E, is
- 51225
- 65625
- 3858
- 3478
Answer
(2) 65625
Solution
[figure]
Z = foot of the perpendicular from V(2, 3) on the directrix: x-22= y-31=- 2(2)+3-65=- 15 Z(1.6, 2.8)
∵ V is the mid-point of ZS focus S(2.4, 3.2)
e2=1- b2a2= 12 a2=2b2
S on E: 5.762b2+ 10.24b2=1 b2=13.12= 32825, a2= 65625
(LR)2= ( 2b2a )2= 4b4a2= 4b42b2=2b2= 65625
Answer: (2)