
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
Given $\triangle ABC$ is an oblique triangle, with the lengths of the sides opposite to angles $A$, $B$, and $C$ being $a$, $b$, and $c$, respectively. If $c\sin A= \sqrt {3}a\cos C$.

(Ⅰ) Find angle $C$;  
(Ⅱ) If $c= \sqrt {21}$, and $\sin C+\sin (B-A)=5\sin 2A$, find the area of $\triangle ABC$.

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