If the four distinct points (4, 6), (-1, 5), (0, 0) and (k, 3k) lie on a circle of radius r, then 10k + r2 is equal to
- 34
- 32
- 35
- 33
Answer
(3) 35
Solution
The circle passes through (0, 0): x2 + y2 + 2gx + 2fy = 0
Through (4, 6): 52 + 8g + 12f = 0 ⇒ 2g + 3f = -13
Through (-1, 5): 26 - 2g + 10f = 0 ⇒ -2g + 10f = -26
Adding: 13f = -39 ⇒ f = -3, g = -2
⇒ Circle: x2 + y2 - 4x - 6y = 0, r2 = g2 + f2 = 4 + 9 = 13
Through (k, 3k): k2 + 9k2 - 4k - 18k = 0
⇒ 10k2 - 22k = 0
⇒ k = 115 (k = 0 gives the point (0, 0) again; the points are distinct)
∴ 10k + r2 = 22 + 13 = 35
Answer: (3)