n.環(huán)帶物(環(huán)形套筒；圓；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. 摘要把緊密相連的圓環(huán)構(gòu)成的圓盤以漸開線展開成等面積的直角三角形，得出了較為直觀的由周長和半徑表示的圓面積公式。
