The point difference method is to set the coordinates p (x 1, y 1) and q (x2, y2) of the two intersections of a straight line and a curve, and then substitute them into the curve equation respectively, and then subtract the two equations to decompose the factors.

Use k = (y 1-y2)/(x 1-x2), x 1+x2 = 2x0, y 1+y2 = 2y0 (where point (x0, y0) is the midpoint coordinate of line PQ).

This paper mainly solves two kinds of problems: the midpoint chord problem and the symmetry problem, which can simplify the calculation.

However, it should be noted that there are two intersections between straight lines and curves.