Integration by substitution allows solving integrals of the form ∫ f(g(x)) g'(x) dx by making the substitution u = g(x) and du = g'(x) dx. This transforms the integral into the simpler form ∫ f(u) du. For example, the integral ∫ cos (x2) 2x dx can be solved using this method by letting f=cos, g=x2, and g'=2x.