Let PQ and MN be two straight lines touching the circle x2 + y2 - 4x - 6y - 3 = 0 at the points A and B respectively. Let O be the centre of the circle and AOB = /3. Then the locus of the point of intersection of the lines PQ and MN is :
- x2 + y2 - 18x - 12y - 25 = 0
- x2 + y2 - 12x - 18y - 25 = 0
- 3(x2 + y2) - 12x - 18y - 25 = 0
- 3(x2 + y2) - 18x - 12y + 25 = 0
Answer
(3) 3(x2 + y2) - 12x - 18y - 25 = 0
Solution
Centre O(2, 3), radius r = 4 + 9 + 3 = 4
Let the tangents at A and B meet at T(h, k). OT bisects AOB AOT = 6, and OAT = 90 .
OT = OA ( /6) = 4 3/2 = 8 3
(h - 2)2 + (k - 3)2 = 643
3(h2 + k2 - 4h - 6k + 13) = 64
3(h2 + k2) - 12h - 18k - 25 = 0
Locus: 3(x2 + y2) - 12x - 18y - 25 = 0
Answer: (3)