Let the image of parabola x2 = 4y, in the line x - y = 1 be (y + a)2 = b(x - c), a, b, c N. Then a + b + c is equal to
- 12
- 8
- 6
- 4
Answer
(3) 6
Solution
Image of (x1, y1) in x - y - 1 = 0:
h - x11 = k - y1-1 = -2(x1 - y1 - 1)2
⇒ h = y1 + 1, k = x1 - 1 ⇒ x1 = k + 1, y1 = h - 1
Since x12 = 4y1: (k + 1)2 = 4(h - 1)
∴ Image: (y + 1)2 = 4(x - 1) ⇒ a = 1, b = 4, c = 1
∴ a + b + c = 6
Answer: (3)