atan(7*x - 1)
d --(atan(7*x - 1)) dx
7 -------------- 2 1 + (7*x - 1)
-98*(-1 + 7*x) ------------------ 2 / 2\ \1 + (-1 + 7*x) /
/ 2 \ | 4*(-1 + 7*x) | 686*|-1 + ---------------| | 2| \ 1 + (-1 + 7*x) / -------------------------- 2 / 2\ \1 + (-1 + 7*x) /