
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
Let the internal angles $A$, $B$, $C$ of $\triangle ABC$ be opposite to the sides $a$, $b$, $c$ respectively, and $c\cos B= \sqrt {3}b\sin C$.  
$(1)$ If $a^{2}\sin C=4 \sqrt {3}\sin A$, find the area of $\triangle ABC$;  
$(2)$ If $a=2 \sqrt {3}$, $b= \sqrt {7}$, and $c > b$, the midpoint of side $BC$ is $D$, find the length of $AD$.

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