Let the point P of the focal chord PQ of the parabola y2 = 16x be (1, -4). If the focus of the parabola divides the chord PQ in the ratio m : n, (m, n) = 1, then m2 + n2 is equal to :
- 17
- 37
- 10
- 26
Answer
(1) 17
Solution
y2 = 16x 4a = 16 a = 4, focus S(4, 0)
Let P = (at12, 2at1) = (4t12, 8t1) = (1, -4) t1 = - 12
For a focal chord, t1t2 = -1 t2 = 2
Q = (4t22, 8t2) = (16, 16)
Focal distance of a point (x, y) = x + a
SP = 1 + 4 = 5, SQ = 16 + 4 = 20
PS : SQ = 5 : 20 = 1 : 4 m = 1, n = 4
m2 + n2 = 1 + 16 = 17
Answer: (1)