The partial derivative of multivariate function can be understood as the generalization of the derivative of univariate function. It is called partial derivative because there are two or more elements in this function, such as X, Y, Z and so on. When we take the partial derivative of x, we can regard other elements such as y and z as constants, so that we can actually understand that this function is a univariate function about x, and the theory and rules are exactly the same as univariate function when taking the derivative. It is also applicable to the derivation of other variables such as y and z, but there are differences in writing in order to distinguish the difference between univariate and multivariate. In fact, writing is just a representative symbol, which can be evolved into multivariate functions according to the idea of monism. The difference between them is that a univariate function can only take the derivative of a variable many times, while a multivariate function can take the derivative of X first, and then take the derivative of Y, Z, etc.