
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
In the arithmetic sequence ${a_n}$, it is known that $a_3=5$, $S_3=12$
(1) Find the general term formula of the sequence ${a_n}$;
(2) Let $b_n=\frac{1}{n a_n}$, find the sum of the first $n$ terms of the sequence ${b_n}$, denoted as $S_n$.

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