Let the length of the focal chord PQ of the parabola y2=12x be 15 units. If the distance of PQ from the origin is p, then 10p2 is equal to .
Answer
72
Solution
[figure]
y2=12x: a=3, focus S(3, 0)
Focal chord inclined at : length =4a cosec2
12 cosec2 =15 2 = 45 = 2
PQ: y= 2(x-3), i.e. 2x-y-6=0 or 2x+y-6=0
p= |-6| 4+1= 6 5 10p2=10 365=72
Answer: 72