If the chord joining the points P1(x1, y1) and P2(x2, y2) on the parabola y2 = 12x subtends a right angle at the vertex of the parabola, then x1x2 - y1y2 is equal to
- 280
- 288
- 292
- 284
Answer
(2) 288
Solution
y2 = 12x a = 3; take P1(3t12, 6t1), P2(3t22, 6t2)
Slope of OP1 = 6t13t12 = 2t1, slope of OP2 = 2t2
OP1 OP2: 2t1 2t2 = -1 t1t2 = -4
x1x2 = 9t12t22 = 9 16 = 144
y1y2 = 36t1t2 = 36 (-4) = -144
x1x2 - y1y2 = 144 + 144 = 288
Answer: (2)