另一个重要的概念是，积分区域。
There is another thing which is the region over which you integrate.
这个图像表达式是用来描述积分区域的。
This is how I want to describe the surface on which I will be integrating.
通常你转换到极坐标系下，有可能积分区域更容易建立。
I mean usually you will switch to polar coordinates either because the region is easier to set up.
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