Let C be the circle of minimum area touching the parabola y = 6 - x2 and the lines y = 3|x|. Then, which one of the following points lies on the circle C ?
- (1, 2)
- (2, 2)
- (1, 1)
- (2, 4)
Answer
(4) (2, 4)
Solution
By symmetry, centre (0, k) on the y-axis, radius r
Touches the parabola at its vertex (0, 6): r = 6 - k
Touches 3x - y = 0: r = |0 - k| 3 + 1 = k2
6 - k = k2 k = 4, r = 2
C : x2 + (y - 4)2 = 4
(2, 4): 4 + 0 = 4 (D) lies on C
(1, 2): 1 + 4 = 5; (2, 2): 4 + 4 = 8; (1, 1): 1 + 9 = 10 (A), (B), (C) do not
Answer: (4)