焦点弦长公式推导过程

设直线和曲线交于两点A(x1,y1) B(x2,y2)
两点间距离公式
|AB|=√[(x1-x2)^2+(y1-y2)^2] y1=kx1+b y2=kx2+b y1-y2=k(x1-x2)
=√[(x1-x2)^2+k^2(x1-x2)^2]
=√ (1+k^2)*|x1-x2|