Let A be the focus of the parabola y2 = 8x. Let the line y = mx + c intersect the parabola at two distinct points B and C. If the centroid of the triangle ABC is ( 73, 43 ), then (BC)2 is equal to :
- 89
- 80
- 32
- 41
Answer
(2) 80
Solution
Focus A(2, 0). Let B(x1, y1), C(x2, y2).
Centroid: 2 + x1 + x23 = 73 x1 + x2 = 5; y1 + y23 = 43 y1 + y2 = 4
y12 - y22 = 8(x1 - x2) m = y1 - y2x1 - x2 = 8y1 + y2 = 2
Put x = y28 in y = 2x + c: y2 - 4y + 4c = 0
x1 + x2 = y12 + y228 = (y1 + y2)2 - 2y1y28 = 16 - 8c8 = 5 c = -3
y2 - 4y - 12 = 0 y = 6, -2
B ( 92, 6 ), C ( 12, -2 )
(BC)2 = 42 + 82 = 80
Answer: (2)