Solution: Parabolic equation can be set as x 2 = 2py according to the meaning of the question.

Then the coordinate of point F is (p/2,0), the directrix equation of parabola is x=-p/2, and the Hou Xian length of AB substituted into parabola is p/4.

In the same way, A 1B 1=p/4 can be obtained.

Because: the length from the focus to the directrix is p/2+p/2 = p.

So A 1F= radical number 65p2/64 = b1f.

So COSA1FB1= (a1f 2+b1f 2-a1ba 2)/2a1f * b1f.

Calculation angle a1FB1= arccos 63/65.

I don't know if it's right. I haven't done math for a long time in senior three.