已知定点F（1,0)和直线l1 y=1,过定点F与直线L相切的动圆圆心为点C

设C(x,y)
|CF| = C和直线l1的距离d,二者平方分别为:
|CF|² = (y - 1)²
d² = (x - 1)² + (y - 0)²
C的轨迹:(x- 1)² = 1- 2y
y = -x²/2 + x
设A(a,-a²/2 + a),B(b,-b²/2 + b)
OA斜率m = -a/2 + 1
OB斜率n = -b/2 + 1
OA垂直OB:(1 - a/2)(1 - b/2) = -1
ab = 4(a + b -2) (1)
AB的方程:
(y - a + a²/2)/(-b²/2 + b - a + a²/2) = (x - a)/(b - a)
整理:2y = (2 - a - b)x + ab = (2 - a - b)x + 4(a + b -2)
= (2 - a - b)(x - 4)
过一个定点(4,0)