曲线的割线是指与弧线有2个公共点的直线。当这两个点不断靠近，并重合为一个点时，这条直线就变成了这条曲线的切线。

在原点处创建单位圆，在P(1,0)处画单位圆的切线。从原点画一条与x轴夹角为θ 的割线，只要θ 不等于90度，割线与切线交于Q点。则θ 的正割等于原点到割线与切线交点的距离，即OQ。

In geometry, a secant line of a curve is a line that (locally) intersects two points on the curve. A chord is an interval of a secant line, the portion of the line that lies within the curve. The word secant comes from the Latin word secare, meaning to cut.