centroid of area

例句：

1. 1. The centroid of the circle is at the center of the circle.
   圆的重心在圆心。
2. 2. The centroid of the triangle is at the midpoint of the bases.
   三角形的重心在底边的中点。
3. 3. The centroid of a rectangle is its diagonal line.
   矩形的重心是它的对角线。