如果取的向量场是处处恒定的，所有点都是平移关系，所以没有散度，因为导数为零。

If you take a vector field that is a constant vector field where everything just translates then there is no divergence involved because the derivatives will be zero.

youdao

在我们开始之前，因为我们需要的是导数，取方程两边的导数。

Before we do that, since what we're after is the derivative, let's take the derivative of both sides.

youdao

在控制方程的推导中，采用物质导数黏弹性本构关系取代通常采用的只对时间取偏导数的黏弹性本构关系。

The governing equation was derived making use of the viscoelastic constitution relation in which, besides the time derivatives, the material derivatives was also taken into account.

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G We can take the derivative of G with respect to how much material there is.

我们可以取,对物质总量的偏导数。

麻省理工公开课 - 热力学与动力学课程节选

So, using those, now, what happens if we take the second derivative of A, the mixed derivative, partial with respect to T and the partial with respect to V.

如果我取A的二阶导数,混合导数，对T偏微分,再对V偏微分。

麻省理工公开课 - 热力学与动力学课程节选

Then, to find the meaning of b, we take one derivative of this, dx/dt, that's velocity as a function of time, and if you took the derivative of this guy, you will find as at+b. That's the velocity of the object.

接下来,为了弄清b的含义,我们取它的一阶导数,dx/dt,得到速度作为时间的函数,如果你对它求导的话,你会得到at+b,这就是物体的速度

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