Question 118
Let $S_n$ denote the sum of the first $n$ terms in a number sequence $\{a_n\}$, satisfying
$$
S_n + a_n = \frac{n-1}{n(n+1)}, \quad n = 1, 2, \dots
$$
Then $a_n = \underline{\hspace{2cm}}$.
$$
S_n + a_n = \frac{n-1}{n(n+1)}, \quad n = 1, 2, \dots
$$
Then $a_n = \underline{\hspace{2cm}}$.