n.环带物(环形套筒；圆；圈)
The disk, which is composed of continuous annuluses, can expand equivalent trangle by asymptotice line.We can obtain the circular area which is visually expressed by circumference and radius. 摘要把紧密相连的圆环构成的圆盘以渐开线展开成等面积的直角三角形，得出了较为直观的由周长和半径表示的圆面积公式。
