直线方程两点式:
(y-y1)/(y2-y1)=(x-x1)/(x2-x1)
y-y1=(y2-y1)(x-x1)/(x2-x1)
y=(y2-y1)(x-x1)/(x2-x1)+y1
即y=x(y2-y1)/(x2-x1)+(x2y1-x1y2)/(x2-x1)