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) /