Answer
$a_{n}=\displaystyle \frac{n}{(n+1)(n+2)}$
Work Step by Step
The numerators progress as
first = 1
second = 2
third =3,
...
n-th = n
The denominators:
first = (2)(3)
second = (3)(4)
third =(4)(5)
4th = (5)(6)
...
n-th = (n+1)(n+2)
So
$a_{n}=\displaystyle \frac{n}{(n+1)(n+2)}$
