
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 $a_{10}=18$, and the sum of the first 5 terms $S_5=-15$.
(I) Find the general term formula $a_n$ for the sequence ${a_n}$;
(II) Find the sum of the first $n$ terms $S_n$ for the sequence ${a_n}$, and find the minimum value of $S_n$.

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