If the shortest distance of the parabola y2=4x from the centre of the circle x2+y2-4x-16y+64=0 is d, then d2 is equal to :
- 16
- 24
- 20
- 36
Answer
(3) 20
Solution
The shortest distance from C to the parabola is measured along the normal at the nearest point, so that normal must pass through the centre C(2, 8).
Normal to y2 = 4x (a = 1) in slope form: y = mx - 2m - m3
Since it passes through (2, 8): 8 = 2m - 2m - m3, so m3 = -8 and m = -2 (the only real normal from C).
Its foot is P(am2, -2am) = (4, 4), hence
d2 = CP2 = (4 - 2)2 + (4 - 8)2 = 4 + 16 = 20
Answer: (3)