Let r be the radius of the circle, which touches x-axis at point (a, 0), a < 0 and the parabola y2 = 9x at the point (4, 6). Then r is equal to .
Answer
30
Solution
Slope of tangent to y2 = 9x: 2y dydx = 9 dydx = 92y = 912 = 34 at (4, 6)
slope of normal = - 43; the centre lies on this normal.
Take centre (4 - 3t, 6 + 4t); its distance from (4, 6) is 5|t| = r.
Circle touches the x-axis r = |6 + 4t|
t > 0: 5t = 6 + 4t t = 6, centre (-14, 30), r = 30, a = -14 < 0 ✓
t < 0: -5t = 6 + 4t t = - 23, centre (6, 103 ), a = 6 > 0 (rejected)
r = 30
Answer: 30