y = (1 + x²)arctanx
y' = (1 + x²) * 1/(1 + x²) * (0 + 2x)arctanx
y' = 1 + 2xarctanx
y'' = 0 + 2[arctanx * x * 1/(1 + x²)]
= 2x/(1 + x²) + 2arctanx