atan(3*x - 1)
3 -------------- 2 1 + (3*x - 1)
-18*(-1 + 3*x) ------------------ 2 / 2\ \1 + (-1 + 3*x) /
/ 2 \ | 4*(-1 + 3*x) | 54*|-1 + ---------------| | 2| \ 1 + (-1 + 3*x) / ------------------------- 2 / 2\ \1 + (-1 + 3*x) /