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If we have a function of more than one independent variable, then we can define a partial derivative with respect to one of the variables, which is simply the derivative of the function with all the other variables fixed, The notation using ∂, which we will use below, tells you it is a partial derivative, For example, suppose we have the function y that depends on the independent variables x and t:
y = A sin(kx – ωt),
where A, k, and ω are constants, Then, "the partial derivative of y with respect to x" is denoted by ∂y/∂x and is found by setting t constant and differentiating with respect to x:
∂y/∂x = kA cos(kx – ωt),
Similarly, "the partial derivative of y with respect to t" is denoted by ∂y/∂t and is found by setting x constant and differentiating with respect to t:
∂y/∂t = –ωA cos(kx – ωt).
Note that the value of either of the partial derivatives depends on both independent variables x and t as well as the constants A, k, and ω.