Two parabolas have the same focus (4, 3) and their directrices are the x-axis and the y-axis, respectively. If these parabolas intersect at the points A and B, then (AB)2 is equal to :
- 384
- 392
- 96
- 192
Answer
(4) 192
Solution
Parabola 1 (directrix y = 0): (x - 4)2 + (y - 3)2 = y2
Parabola 2 (directrix x = 0): (x - 4)2 + (y - 3)2 = x2
At common points: x2 = y2 y = x or y = -x
y = -x: (x - 4)2 + (x + 3)2 = x2 x2 - 2x + 25 = 0, no real root.
y = x: (x - 4)2 + (x - 3)2 = x2 x2 - 14x + 25 = 0
x1 + x2 = 14, x1x2 = 25
(x1 - x2)2 = 196 - 100 = 96
A and B lie on y = x (AB)2 = 2(x1 - x2)2 = 192
Answer: (4)