Multivariable calculus

multiple variable calculus, that is to say: a function which have multiple parameters, and you must do calculus with it.

Multi-variable calculus (also known as multivariate calculus) is the extension of calculus in one variable to calculus with functions of several variables: the differentiation and integration of functions involving multiple variables, rather than just one.

Why we have to learn it? Well, simply enough, when you trying to get an object's volume, all you have to do is find its 2-D area formula, then do integration with its height.