Dx is the differential of X, which can also be understood as "infinitesimal", that is, a small segment of the independent variable X, or a small segment on the X axis (small means that there is nothing smaller than it, but it is not equal to zero). The geometric significance of differential is that the curve can be approximated locally by a straight line, and the error is only an infinitesimal amount about dx, which can be ignored. Usually, the increment Δ x of the independent variable x is called the differential of the independent variable, which is denoted as dx, that is, dx = Δ x.