centroid of a free tree

例句：

1. 1.The centroid of the circle is at its center.
   圆的重心在其中心。
2. 2.The team's centroid is in the center of the field.
   队伍的重心位于场地中心。
3. 3.The weight distribution around the centroid is symmetrical.
   围绕重心的重量分布是对称的。