f(x) = ATAN(1/√(x^2 + 1))
f(x) = ATAN((x^2 + 1)^{- 1/2})
f'(x) = 1/(((x^2 + 1)^{- 1/2})^2 + 1)·(- 1/2·(x^2 + 1)^{- 3/2})·(2·x)
f'(x) = 1/(1/(x^2 + 1) + 1)·(- 1/2·(x^2 + 1)^{- 3/2})·(2·x)
f'(x) = (x^2 + 1)/(x^2 + 2)·(- 1/(2·(x^2 + 1)^{3/2}))·(2·x)
f'(x) = - x/((x^2 + 2)·√(x^2 + 1))