>>> def f(n):
...     s = 0
...     for i in range(1, n + 1):
...         s += 1 / (2 * i * (2 * i + 1) * (2 * i + 2))
...     return s
... 
>>> f(1)
0.041666666666666664
>>> f(2)
0.049999999999999996
>>> f(3)
0.05297619047619047
>>>