这就是切平面的，或者说这个曲面的法向量。

OK, and that's going to be the normal vector to the surface or to the tangent plane.

youdao

现在，假设曲面上有两条相交切线，那么这两条切线可以确定一个切平面，我们来看看这个平面是怎么搞出来的。

Now, if I have two lines tangent to the surface, well, then together they determine for me the tangent plane to the surface. Let's try to see how that works.

youdao

垂直于曲面也意味着垂直于它的切平面，垂直于曲面也意味着垂直于它的切平面。

Being perpendicular to the surface means that you are perpendicular to its tangent plane.

youdao