Dx refers to the differential of X, that is, the tiny variable of X, which can be understood as the variation of X △x=x2-x 1.

Dt refers to the differential of x, that is, the tiny variable of t, which can be understood as the variation of x △t=t2-t 1.

It can be said that the instantaneous velocity is the ratio of displacement to time in a short time before or after a certain moment.