We take as the second derivative formula.

取作为二次微商计算式。

So, what does the second derivative test say?

那么二阶导数判定是怎样的呢？

And all we did, further, is take that second derivative.

总而言之，我们所做的就是求二阶导数。

So, it's the second derivative of a position vector.

它是位置向量的二阶导。

The second derivative, they say, is turning positive.

他们说二阶导数转为正值了。