Then, if I choose any vector in that tangent plane.

那么如果我选择了任何位于切平面的向量。

Let's think about the tangent plane with regard to a function f.

我们来考虑,关于函数f的一个切平面。

If we were moving on the tangent plane, this would be an actual equality.

如果我们在切平面上移动,这将会是一个真正的等式。