Fundamental Theorem of Calculus !!

          The fundamental theorem of calculus is (intuitively) in the claim that the derivation and integration are inverse operations function. This means that any continuous integrable function verifies that the derivative of its integral is equal to itself. This theorem is central to the branch of mathematics called mathematical analysis or calculus.

          The theorem is essential because until then the estimate of area-integral-which had been working since Archimedes was a branch of mathematics that is still separate differential calculus that had been developed by Isaac Newton, Isaac Barrow and Gottfried Leibniz in the eighteenth century and led to concepts such as derivatives. The integrals were investigated as ways to study areas and volumes, until this point in history both branches converged, the study demonstrated that the "area under a function" was intimately connected to differential calculus, resulting in the integration, the inverse operation referral.

         A direct consequence of this theorem is the rule of Barrow, sometimes called the second fundamental theorem of calculus, which calculates the integral of a function using the indefinite integral of the function to be integrated.

In all areas of mathematics there fundamental theorems, which are the core of some mathematical branch, so the same as a simple comment, the calculation has three funadamntales concepts, which are set, functions and limitations.
And it is from them that is based on calculation for better study and cocepción, because all three are vastly larger and a great contribution to mathematics.