atan(a*x)
a --------- 2 2 1 + a *x
3 -2*x*a ------------ 2 / 2 2\ \1 + a *x /
/ 2 2 \ 3 | 4*a *x | 2*a *|-1 + ---------| | 2 2| \ 1 + a *x / --------------------- 2 / 2 2\ \1 + a *x /