the unit normal vectors 单位正常矢量
And the way it will work is that you have to choose an orientation of a surface, which just means choosing one of the two possible unit normal vectors.
具体方法是，先给曲面定向，也就是选择两个单位法向量中的一个。
The first unit about cross-products,normal vectors and so on is actually useful now.
第一单元学得叉积、法向量这些,现在就有用了。
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