Let me give you an idea. Since the circle Q is tangent to both the X axis and the straight line y=mx, that is, Q falls on the bisector of the angle AOE. Then you connect the tangent points to get two 90 degrees, subtract two 90 degrees and one 60 degrees from 360 degrees to get 120 degrees, and then prove it by the vertical diameter theorem (bisecting the central angle); The other angle is 60 degrees. It's only three o'clock.

The coordinate of d can be set to (k, 2r), and the parabola can be written as the vertex y=a(X-k)? +2r .

Substitute y= 1 to find the two-point coordinates of m and n, calculate half the length of MN line segment, and then use projective theorem.