
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 $A$, $B$, and $C$ are denoted as $a$, $b$, and $c$ respectively. It is given that $\frac{b}{a}= \frac{\sin B}{\sin 2A}$.

$(1)$ Find the value of angle $A$;  

$(2)$ If $b=2\sqrt{3}$ and the area of $\triangle ABC$ is $\frac{3\sqrt{3}}{2}$, find the value of $a$.

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