If AB is a parabola y2 = 2px (p >; 0), and A 1B 1 is AB's photography on the alignment. What is the angle of A 1FB 1?

The directrix is x=-p/2. According to the definition of parabola, the sum of the distances from the two ends of focus chord to the focus, that is, the chord length, is equal to the sum of the distances from these two points to the directrix.

This problem actually solves the difference between the y values of two points. The focal chord length is p+x 1+x2, and the angle between focus chord and the x axis is θ, then a1b1= (p+x1+x2) sinθ = 2p/(sinθ).