Definition:Partial Derivative

Definition
Given a function of multiple independent variables $f \left({x_1, \ldots, x_n}\right)$, the partial derivative with respect to $x_i$ is denoted and defined as:


 * $\dfrac{\partial f}{\partial x_i} = \dfrac{\mathrm d g}{\mathrm d x_i}$

where:
 * $g \left({x_i}\right) = f \left({x_1, \ldots, x_i, \dots, x_n}\right)$
 * $\dfrac{\mathrm d g}{\mathrm d x_i}$ is the derivative of $g$ with respect to $x_i$
 * all the $x_j, j \ne i$ are considered as constant.

Other notations are:


 * $\dfrac {\partial z}{\partial x_i}$
 * $\dfrac {\partial f}{\partial x_i}$


 * $f_{x_i} \left({x_1,x_2,\cdots,x_n}\right)$
 * $f_{x_i}$
 * $z_{x_i}$

where $z = f\left({x_1,x_2,\cdots,x_n}\right)$.

When spoken, $\dfrac {\partial y}{\partial x}$, "the partial derivative of $y$ with respect to $x$" is often shortened to "partial $y$ partial $x$", or "del $y$ del $x$".

Also see

 * Gradient