So, remember, when we set up the divergence theorem, we need the normal vectors to point out of our region, which means that on the top surface, n But, on the bottom face, we want n pointing down.

当我们建立散度定理时，我们要使法向量指向区域外，即在上表面时，要指向上方。，we，want，n，pointing，up。,但是，在下表面，要使n指向下面。

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n We're interested in x divided by n. So let's divide both the top and the bottom by n squared.

我们感兴趣的是x除以,所以同时把分子分母除以n平方。

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The N, B and Nicotine content in tobacco leaves drop significantly in the order of top leaves, middle leaves and bottom leaves, while K, Fe and Reducing Sugar increase significantly in the same order;

氮、硼、烟碱含量从上部烟叶到下部烟叶显著地依次降低，而钾、铁、还原糖则从上部烟叶到下部烟叶显著地依次升高；

youdao

The N, B and Nicotine content in tobacco leaves drop significantly in the order of top leaves, middle leaves and bottom leaves, while K, Fe and Reducing Sugar increase significantly in the same order;

氮、硼、烟碱含量从上部烟叶到下部烟叶显著地依次降低，而钾、铁、还原糖则从上部烟叶到下部烟叶显著地依次升高；

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