directrix of a conic

例句：

1. 1.The directrix of the cone is perpendicular to the base of the cone.
   圆锥的轴垂直于圆锥底面。
2. 2.The directrix of a parabola is the line that divides the graph into two equal halves.
   抛物线的直母线是将图形分成两个相等半部分的线。
3. 3. The directrix of a cylinder is the line that divides the top and bottom of the cylinder.
   圆柱体的直母线是将圆柱体顶部和底部分开的线。