In fact, the introduction of line integral is essentially a generalization and generalization of definite integral. Because the integral range of definite integral is a straight line, but this

A straight line lies on the coordinate axis. So what if this straight line is not on the coordinate axis? Further, what if it is a curve instead of a straight line? This definite integral cannot be solved. Therefore, curve integral should be introduced to extend the integral range. As for solving curve integral, it is because it is the easiest to solve definite integral among all line integrals. Of course, we should turn a difficult problem into a simple one. But this transformation is only for evaluation, and it cannot explain the geometric or physical meaning of curve integral.

Similarly, surface integral is a generalization of double integral in the integral range. You can think for yourself why.

I hope this will help you understand. .