envelope of curve

例句：

1. 1. The shape of theenvelope is determined by the lengths ofthe sides.
   信封的形状由边长决定。
2. 2. Theenvelope of a circle is a circle witha single hole in it.
   圆的包络是一个带有单个孔洞的圆。
3. 3. Theenvelope of a curved surface is thesmallest region that contains all points on thesurface.
   曲面的包络是包含曲面上所有点的最小区域。