atan(5*x - 1)
5 -------------- 2 1 + (5*x - 1)
-50*(-1 + 5*x) ------------------ 2 / 2\ \1 + (-1 + 5*x) /
/ 2 \ | 4*(-1 + 5*x) | 250*|-1 + ---------------| | 2| \ 1 + (-1 + 5*x) / -------------------------- 2 / 2\ \1 + (-1 + 5*x) /