公切线的英文：
common tangent
adj. 普通的；一般的；通常的；通俗的；公共的，共有的；共同的
n. 普通；平民；公共地
international common
国际公地
Commonness is happiness.
平凡是福。
A common danger causes common action
同仇敌忾
The series uses common components and common interior electronics.
该系列采用普通的元件和内部电子器件。
Common fame is liar
盛名之下,其实难副
n. 正切；直线区间；突兀的转向；突然转移话题
The direction is tangent to the trajectory.
方向就是轨道的切线方向。
The tangent is a relatively important concept in trigonometry.
The graph of a concave function is always below its tangent.
一个凹函数的图象总在它的切线的下方。
Arrowheads on the lines indicate the direction in which the tangent is to be drawn
电力线上的箭头表示所要画的切线方向。
Don't fly off at a tangent, we are talking about the money problem.
不要扯别的，我们谈的是那笔钱的问题。