Integration!

 Integration is a part of calculus and later real analysis, which comes after differentiation and is technically the opposite of it. Differentiation means breaking up a part into minute small pieces, while integration means adding all those parts to get the whole piece.
We consider integration to be very easy since it is taught at a very nominal level in schools.
The real challenge comes when we start dealing with it in our semesters in college. We get to understand the importance of Riemann integrals and gradually go to the very depth of the formulas we used to blindly apply to get ourselves marks in schools.
Unfortunately, in many colleges, integration is not taught in a systematic manner, and hence before knowing and experiencing Riemann integrals, we start applying reduction formulae and jump to definite integration.

But I have been lucky to have basics get cleared while remaining in class 12, by some wonderful teachers, who made me fall in love...
Am sharing some of their notes, and collegiates please take a look, it will be very helpful✌

https://drive.google.com/file/d/1ITColL7_iyNmovRKVqFPYy1BvRHMzjDm/view?usp=sharing