The shortest distance between the curves y2 = 8x and x2 + y2 + 12y + 35 = 0 is:
- 2 3 - 1
- 2 2 - 1
- 3 2 - 1
- 2
Answer
(2) 2 2 - 1
Solution
Circle: x2 + (y + 6)2 = 1
centre (0, -6), radius 1.
The shortest distance lies along the common normal, i.e. the normal to the parabola that passes through the centre.
For y2 = 8x (a = 2), normal at (2t2, 4t): y = -tx + 4t + 2t3
Passing through (0, -6): -6 = 4t + 2t3
t3 + 2t + 3 = 0 (t + 1)(t2 - t + 3) = 0
t = -1 (only real root)
foot of normal (2, -4)
Distance from centre = (2 - 0)2 + (-4 + 6)2 = 2 2
shortest distance = 2 2 - 1
Answer: (2)