圆面积公式是一种定理定律。为圆周率*半径的平方，用字母可以表示为：S=πr²或S=π*（d/2)²。（π表示圆周率（3.1415926……），r表示半径，d表示直径）。
统计典型区域中毛细管个数，参照圆面积公式得到纱线中毛细管当量半径。
According to the area of capillaries, the number of capillary and the formula of circle, the equivalent radius of the five kinds of the yarns was gotten.
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。
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.
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