这个曲面积分，不是同一个向量场。
OK, and that surface integral, well, it's not for the same vector field.
其实是要对一个与其无关的向量场积分，其实是要对一个与其无关的向量场积分。
We are just integrating a vector field that has nothing to do with that.
你懂得向量微积分，但你不记得如何做长除法。
You know vector calculus but you can't remember how to do long division.
你懂得向量微积分，但你不记得如何做长除法。
You know vector calculus but you can't remember how to do long division.
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