为了找到带法向量的面元，我就取这两个的叉积。
To find the surface element together with a normal vector, I would just take the cross-product between these guys.
目前比较成熟的数值方法有：蒙特卡罗方法，二叉树方法，有限元和有限差分方法。
At present, the most commonly used methods include: Monte Carlo methods, binomial tree, finite element and finite difference methods.
作者设计并实现了一个基于多变元逐步回归的二叉树分类器。
A binary tree classifier based on multivariate step-wise regression was designed and implemented.
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