atan(7*x)
d --(atan(7*x)) dx
7 --------- 2 1 + 49*x
-686*x ------------ 2 / 2\ \1 + 49*x /
/ 2 \ | 196*x | 686*|-1 + ---------| | 2| \ 1 + 49*x / -------------------- 2 / 2\ \1 + 49*x /