接着，创建一个spl函数，它将返回给定半径的圆的面积。
Next, create an SPL function that returns the area of a circle given its radius.
本示例展示如何对圆的面积创建函数索引。
This example shows how to create a functional index on the area of a circle.
DXF和DWG文件格式还支持弧和圆，您可以在输出格式中通过创建多个小线段来近似模拟圆或弧。
The DXF and DWG file formats also support arcs and circles, which you can approximate in your output formats by creating multiple small line segments approximating the circle or arc.
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