Because the binary function is related to both X and Y variables, it is possible that the first derivative of U with respect to X (in this case, Y is regarded as a constant) is constant. At this time, M0 reached the maximum. For example, y in U(x, y) is a constant, and multivariate functions are different in finding the extreme value domain. Conversely, the definition can be clarified. This is the complete works of Li Yongle.