What does D mean in derivatives and integrals?

In the (1) derivative, y = f (x) f' (x) = dy/dx. What does d mean? Can it be written as dxf(x)=dy?

Answer: d means differential, which can be written as dxf(x)=dy.

(2) In addition, in the ∫xdx integral, what does dx mean? Sometimes it can be written as du, but u is a formula, such as du = x 2dx.

A: dx stands for integration with X, and the latter U is only an intermediate variable. After replacement, it is still integrated with X.