Book:Murray R. Spiegel/Mathematical Handbook of Formulas and Tables/Chapter 13/Partial Derivatives

Partial Derivatives
Let $f \left({x,y}\right)$ be a function of the two variables $x$ and $y$. Then we define the partial derivative of $f \left({x,y}\right)$ with respect to $x$, keeping $y$ constant, to be:
 * $13.58-59$: Definition of Partial Derivative


 * $13.58$: $\dfrac {\partial f} {\partial x} = \lim_{\Delta x \mathop \to 0} \dfrac {f \left({x + \Delta x, y}\right) - f \left({x, y}\right)} {\Delta x}$

Similarly the partial derivative of $f \left({x,y}\right)$ with respect to $y$, keeping $x$ constant, is defined to be:
 * $13.59$: $\dfrac {\partial f} {\partial y} = \lim_{\Delta y \mathop \to 0} \dfrac {f \left({x, y + \Delta y}\right) - f \left({x, y}\right)} {\Delta y}$

Partial derivatives of higher order can be defined as follows.
 * $13.60-61$: Definition of Second Partial Derivative
 * $13.60.1$: $\dfrac {\partial^2 f} {\partial x^2} = \dfrac \partial {\partial x} \left({\dfrac {\partial f} {\partial x} }\right)$
 * $13.60.2$: $\dfrac {\partial^2 f} {\partial y^2} = \dfrac \partial {\partial y} \left({\dfrac {\partial f} {\partial y} }\right)$
 * $13.61.1$: $\dfrac {\partial^2 f} {\partial x \partial y} = \dfrac \partial {\partial x} \left({\dfrac {\partial f} {\partial y} }\right)$
 * $13.61.2$: $\dfrac {\partial^2 f} {\partial y \partial x} = \dfrac \partial {\partial y} \left({\dfrac {\partial f} {\partial x} }\right)$

By Partial Differentiation Operator is Commutative for Continuous Functions, $13.61.1$ and $13.61.2$ will be equal if the function and its partial derivatives are continuous, that is, in such case the order of differentiation makes no difference.

The differential of $f \left({x,y}\right)$ is defined as:
 * $13.62$: Definition of Differential for Multi-Variable Functions: $\mathrm d f = \dfrac {\partial f} {\partial x} \mathrm d x + \dfrac {\partial f} {\partial y} \mathrm d y$

where $\mathrm d x = \Delta x$ and $\mathrm d y = \Delta y$.

Extension to functions of more than two variables are exactly analogous.