1. The problem statement, all variables and given/known data Integrate x^3*sqrt(x^2 +1) 2. Relevant equations The problem is to be solved by using substitution 3. The attempt at a solution To be honest i'm at a complete stump here, I've tried most values for U. My first guess was x^3, since du/dx = 3x^2, but I can't substitute this into a root, can I? Choosing x^2+1 as U is also moot, as the derivative equals 2x and by substituting you are still left with x^2 on the left side of the equation. So I thought perhaps the best idea would be to substitute all of sqrt(x^2 +1), as the derivative becomes x/(sqrt(x^2+1)) = x/u, but even then, i'm stuck with x^2 by substituting. I've also tried all sorts of ways to rewrite the equation (such as x^3*(x^2+1)^0.5, however, with no luck). I'm really starting to wonder how i'm supposed to integrate this function by use of substitution, anyone got any thoughts that could push me in the right direction? Thanks.