You take the flux, multiply it by the cross section, average it, integrate it over the whole volume.

你拿流量乘以横截面，平均它，乘以它除以总容积。

youdao

But, also, if you sum the flux through the surface of each little piece, well, you will see that you will be integrating twice over each of these vertical cuts.

如果对这些小块表面通量求和，就会看到，这些垂直切口积分了两次。

youdao

Then I can actually -- --replace the line integral for flux by a double integral over R of some function.

那么我就能名正言顺地,用R上的某个函数的二重积分来替代通量的线积分。

youdao

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