You can use the tangent equation: y 2 = 2px. The tangent equation at (X 1, Y 1) is y*y 1=p*(x+x 1). If you don't understand, you can take the derivative by analogy with x 2 = 2py (or y 2). Get two straight lines

y * y 1 = p *(x+x 1)； y*y2=p*(x+x2)

The product of slopes is y 1 * y2/p 2. There is a conclusion in the textbook that the product of the straight line passing through the focus and the ordinate of the intersection of parabola y 2 = 2px is y 1 * y2 =-p 2, so the two straight lines are vertical, and C is on the circle with AB as the diameter. You can guess that the answer is the root number a * b.

After calculation, it is easy to find the coordinates of C (-p/2, 1/2(y 1+y2)).

Given above, the slope of AB is y1-y2/x1-x2 = 2p/y1+y2, and the product of the slope of CF and the slope of AB is-1.

Using the projective theorem, in the right triangle ABC, CF is perpendicular to AB, and CF is the root number A * B. ..

There is also a method of subsection calculation, which uses the formula of the distance between two points, | cf | = root sign (p 2+1/4 (y1+y2) 2), resulting in y 2 = 2px, and the formula of joint focal radius is a = x 1+p/2.