圆锥曲线的焦点弦问题是解几教学的一个重点与难点，也是各类考试的热点．解答此问题，不仅演算繁长，而且稍不留心，就出差错．为此，本文利用极坐标推导出圆锥曲线在直角坐标系中的焦点弦长度的一种表达形式─—三角形式．现说明如下：定理AB是经过椭圆b~2x~2＋a~2y~2=a~2b~2 The focal chord problem of the conic curve is a key and difficult point in solving the teaching, and it is also a hot spot for all kinds of examinations. To answer this question, not only the calculations are lengthy, but also a little carelessness makes mistakes. For this reason, this paper uses polar coordinates to derive an expression of the focal chord length of a conic section in a rectangular coordinate system--triangle. The description is as follows: Theorem AB is passing through the ellipse b~2x~2+a~2y~2=a~2b~2