
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given the sequence ${a_n}$ with the sum of its first $n$ terms denoted as $S_n$, and $a_n= \frac {n\cdot 2^{n}-2^{n+1}}{(n+1)(n^{2}+2n)}\ (n\in\mathbb{N_+})$, find the expression for $S_n$.

Present the answer in LaTex format: \boxed{Your answer}
user