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# Differential

It's quite simple, if I give you an expression:

$$
z = x y
$$

You will get:

$$
\begin{align\*}
dz &= (xy)^\prime\_x dx + (xy)^\prime\_y dy
\ \\
dz &= \frac{\partial z}{\partial x} dx + \frac{\partial z}{\partial y} dy
\ \\
dz &= y \cdot dx + x \cdot dy
\end{align\*}
$$