在散度定理中的约定是,将曲面的定向取为外法线的方向。

The convention in the divergence theorem is that we orient the surface with a normal vector pointing always outwards.

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根据渐近曲线的方程，具体给出主法线曲面的一族非直线的渐近曲线。

According to the equation of asymptotic curves, a family of nonlinear asymptotic curves are given to the principal normal surface.

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这四个方面分别是：（1）已知数据点的正投影和曲面上数据点处的法线方向，构造自由曲面；

The four aspects is as follow:(1) Modelling a free curved surface from the orthogonal projections and the normal vectersof controlling points.

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