The substitution rule, a.k.a. integration by substitution, or u-substitution, is the most common technique of integration. If F′=f, then
∫f(u)dudxdx=F(u)+C.
By choosing a suitable function u, we can often convert hard integrals into much easier integrals that we know how to evaluate. Unfortunately, there is no magic formula for deciding what u should be — this is a toolkit, not a recipe.