当平行于梯形底的直线在梯形内部和外部平移的时候,我们可以得到下述一个重要性质。 定理:在平行于梯形底的任一条直线上,被夹在一条腰(或其延长线)和一条对角线(或其延长线)之间的线段与被夹在另一条腰(或其延长线)和另一条对角线(或其延长线)之间的线段长相等。 When the line parallel to the bottom of the trapezoid moves inside and outside the trapezoid, we can get one of the following important properties. Theorem: On any straight line parallel to the bottom of the trapezoid, the line segment sandwiched between one waist (or its extension) and one diagonal line (or its extension) is sandwiched between the other waist (or its extension) The line length between the line and another diagonal line (or its extension line) is equal.