Integral, what does it mean to always take a dx or ds in physics?

Is to differentiate. The basic idea of differentiation is to change into invariance.

For example, find the variable force F(x)=kx to do work.

Let's take a very short distance dx, where f can be regarded as a constant, so the work done by internal force at this distance is equal to the work of yuan: dA=F(x)dx=kxdx.

The total * * * is the sum of many such DAs. That is, just synthesize the above formulas.