我们说一个函数是可微的，如果这些偏导数存在。

And, we say that a function is differentiable if these things exist.

youdao

未知量是一个函数，这个方程，将把函数的偏导数联系起来。

So the unknown is a function, and the equation will relate the partial derivatives of that function to each other.

youdao

但是有一件事情，我必须指出来，在现实中，很多函数的偏导数之间，存在着很好的关系。

But one thing at a time. I wanted to point out to you that very often functions that you see in real life satisfy many nice relations between the partial derivatives.

youdao