Suppose AB is a focal chord of the parabola y2 = 12x of length l and slope m < 3. If the distance of the chord AB from the origin is d, then l d2 is equal to .
Answer
108
Solution
y2 = 12x a = 3, focus S(3, 0); let the chord make angle with the x-axis (m = )
Length of focal chord: l = 4a cosec2 = 12 cosec2
Chord through S: d = OS = 3 d2 = 9 2
l d2 = 12 cosec2 9 2 = 108 (independent of m)
Answer: 108