Which substitution is most appropriate for evaluating ∫6x2x3+1dx\int \frac{6x^2}{\sqrt{x^3+1}} dx∫x3+16x2dx?
u=x2u = x^2u=x2
u=x3+1u = x^3+1u=x3+1
u=x3+1u = \sqrt{x^3+1}u=x3+1
u=6x2u = 6x^2u=6x2