圆面积是指圆形所占的平面空间大小，常用S表示。圆是一种规则的平面几何图形，其计算方法有很多种，比较常见的是开普勒的求解方法，卡瓦利里的求解方法等。
这样，有90％以上的光落在第二暗环所包围的圆面积之内。
Thus more than 90 per cent of the light is contained within the circle bounded by the second dark ring.
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.
如果数据集较大，或者经常发出请求圆面积的查询，那么创建函数索引将有助于提高性能。
This is beneficial when the dataset is large and the frequency of queries that request the area of a circle is high.
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