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
For y2 = 16x, 4a = 16, so a = 4 and the focus is S(4, 0).
Let P = (at12, 2at1) = (4t12, 8t1) = (1, -4), which gives t1 = - 12.
For a focal chord t1t2 = -1, so t2 = 2 and Q = (4t22, 8t2) = (16, 16).
The focal distance of a point (x, y) is x + a, so SP = 1 + 4 = 5 and SQ = 16 + 4 = 20.
Hence PS : SQ = 5 : 20 = 1 : 4, i.e. m = 1, n = 4.
m2 + n2 = 1 + 16 = 17
Answer: (1)