Taking the derivative of a single variable function is easy, because you simply have to differentiate with respect to the only variable in the function, and therefore, there's only one derivative. Multivariable functions are slightly more complicated, because you have to find a different derivative for each of the variables in the function. These are called partial derivatives, and you'll have one partial derivative for each of the variables in the function. For example, given a function z defined in terms of x and y, you'll have two partial derivatives, one for each of the variables x and y. You'll call them the partial derivative of z with respect to x and the partial derivative of z with respect to y.