我们还学过微分的链式法则，也就是用其他量来代替这些偏导数。

So, we've learned about differentials and chain rules, which are a way of repackaging these partial derivatives.

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现在可以看到，全微分里面的这些偏导数系数，都可以用一个变量表示出来。

Now you see how the total differential accounts for, somehow, all the partial derivatives that come as coefficients of the individual variables in these expressions.

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采用了压缩性的坐标变换后，推导得到了五个一阶导数的微分方程组。

Using a compressibility coordinate transformation, a set of the first derivative differential equations has been derived.

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