we integrate the derivative du/dx = 2 x we also get the original u = x^2.
Differentiation is the inverse of integration, an observation known as the
Fundamental Theorem of the Calculus and one of the most important theorems
in all mathematics.
Without Calculus there would be no satellites in orbit, no economic theory and
statistics would be a very different discipline. Wherever change is involved, there
we find Calculus.

the condensed idea

Going to the limit