
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 $\triangle ABC$, the sides opposite to angles $A$, $B$, $C$ are respectively $a$, $b$, $c$, given that the vectors $\vec{m}=(\cos A,\cos B)$, $\vec{n}=(a,2c-b)$, and $\vec{m}\parallel \vec{n}$.  
(I) Find the magnitude of angle $A$;  
(II) Find the maximum value of $\sin B + \sin C$ and determine the shape of $\triangle ABC$ at this time.

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