And that is an approximation for partial derivative.

这就是偏导的近似值。

OK, so that's the definition of a partial derivative.

好的，那就是偏导数的定义。

So, at that point, the partial derivative is zero with respect to x.

因此这点上x的偏导为零。

We will replace the partial derivative by an ordinary derivative.

我们可用普通微商代替偏微商。

Well, we have seen that it is given by the partial derivative f sub x.

我们已经发现,它由f对x的偏导数给出。