Integration problem

Question

I was working through this question and understand it up to the point where 1/3 becomes 1/48 and was wondering how this happens.

I have u = x^3 + 3x + 5

and dx = du/3(x^2+1)

so substituting, the x^2+1 cancels and gives

but then it goes to:

and im not sure why and don't know how to continue after this. Thank you.

Answer 1

Substitution method

Now the integral becomes

Comments

Thank you!