Let PQ be a chord of the parabola y2 = 12x and the midpoint of PQ be at (4, 1). Then, which of the following point lies on the line passing through the points P and Q ?
- (3, -3)
- ( 12, -20 )
- (2, -9)
- ( 32, -16 )
Answer
(2) ( 12, -20 )
Solution
Chord with mid-point (x1, y1): T = S1 ⇒ yy1 - 6(x + x1) = y12 - 12x1
(4, 1): y - 6(x + 4) = 1 - 48 ⇒ PQ: y = 6x - 23
(A): 6(3) - 23 = -5 −3 ⇒ (A) not on PQ
(B): 6 12 - 23 = -20 ⇒ (B) lies on PQ
(C): 6(2) - 23 = -11 −9 ⇒ (C) not on PQ
(D): 6 32 - 23 = -14 -16 ⇒ (D) not on PQ
Answer: (2)