Let the directrix of the parabola P : y2=8x, cut x-axis at the point A. Let B( , ), >1, be a point on P such that the slope of AB is 3/5. If BC is a focal chord of P, then six times the area of ABC is :
- 80
- 160
- 174
- 192
Answer
(2) 160
Solution
[figure]
y2=8x 4a=8, a=2: directrix x=-2 A(-2, 0), focus F(2, 0)
Let B(2t2, 4t); slope of AB = 4t2t2+2= 35
3t2-10t+3=0 t=3 or 13
=2t2>1 t=3 B(18, 12)
BC is a focal chord tC=- 1t=- 13 C ( 29, - 43 )
Area = 12 |x1(y2-y3)+x2(y3-y1)+x3(y1-y2) |
= 12 |-2 (12+ 43 )+18 (- 43-0 )+ 29(0-12) |
= 12 |- 803-24- 83 |= 12 1603= 803
6 Area =160
Answer: (2)