atan(5*x)
5 --------- 2 1 + 25*x
-250*x ------------ 2 / 2\ \1 + 25*x /
/ 2 \ | 100*x | 250*|-1 + ---------| | 2| \ 1 + 25*x / -------------------- 2 / 2\ \1 + 25*x /