dz=？ z/？ xdx+？ z/？ Ydy. Definition of differentiation in mathematics: From the function B=f(A), two groups of numbers A and B are obtained. In A, when dx approaches itself, the limit of the function at dx is called the differential of the function at dx, and the central idea of the differential is infinite division. Differential is the linear main part of function change. One of the basic concepts of calculus. The solution of dz is: dz=? z/？ xdx+？ z/？ Ydy is a fully differential formula. z/？ X is the partial derivative of z to x, z/? Y is the partial derivative of z to y, and Dz is the differential of function value, which is the main part of the change of function value. So it is the multiplication of two partial derivatives and the differentiation of their respective independent variables, and then they are added.