The reverse chain rule is also called "u-substitution." This is a way to find an integral, but it only works when the given equation is set up in a special way that lets us include both g(x) and its derivative g'(x) in the same equation. Using the integration chain rule, you can find the sum of a very large scale. It's easy to figure out how to solve the small equation if you understand how it works and have already solved some of the questions. But when the equation is big or hard to understand, the integration chain rule is used to find the answer.