arithmetical mean diameter

例句：

1. 1.The average diameter of the tree is about 30 feet.
   这棵树的平均直径大约是30英尺。
2. 2.The diameter of the pipe is 1/4 inch larger than the other one.
   这个管子的直径比另一个管子大1/4英寸。
3. 3. The diameter of the hole in the wall is too small for a coat hanger.
   墙上的洞太小，不适合挂衣架。