另一种选择，也就是我要选的那种，是转换坐标系，让这个点变成原点。
The other option, which is the one I will choose, is to change the coordinate so that this point become the origin.
但还是可以建立这个积分，然后转换到极坐标系下去求结果。
But still we could set this up and then switch to polar coordinates to evaluate this integral.
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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