Talking about integrals

Limit is the foundation of the whole calculus, and all the concepts of calculus are based on limit.

In unary function, differentiability and differentiability are equivalent.

Indefinite integral is the inverse of derivative operation.

The definition of definite integral is the limit of integral sum, and the original meaning is the limit problem; The derivative of integral (that is, integral function) has a variable upper limit, which is the integrand function, so the problem of definite integral is also the problem of finding the original function. In this way, differential and integral are a pair of reciprocal operations.