Let a circle of radius 4 pass through the origin O, the points A(- 3a, 0) and B(0, - 2b), where a and b are real parameters and ab 0. Then the locus of the centroid of OAB is a circle of radius
- 73
- 113
- 53
- 83
Answer
(4) 83
Solution
AOB = 90 (A on x-axis, B on y-axis), so AB is a diameter of the circle.
AB = 2 4 = 8
3a2 + 2b2 = 64 ...(i)
Centroid (h, k): h = - 3a3, k = - 2b3
3a2 = 9h2 and 2b2 = 9k2
Substituting in (i): 9h2 + 9k2 = 64
h2 + k2 = 649
locus is a circle of radius 83
Answer: (4)