3 -16*x*a ------------ 2 / 2 2\ \4*a + x /
3 -16*x*a ------------ 2 / 2 2\ \x + 4*a /
-16.0*x*a^3/(x^2 + 4.0*a^2)^2
3 -16*x*a -------------------- 4 4 2 2 x + 16*a + 8*a *x