# Differential

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

$$
z = x y
$$

You will get:

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

