
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 the following four propositions:  
① There exists a real number $\alpha$ such that $\sin\alpha \cdot \cos\alpha = 1$;  
② $f(x) = -2\cos\left( \frac {7\pi }{2} - 2x\right)$ is an odd function;  
③ $x = - \frac {3\pi }{8}$ is a symmetry axis of the graph of the function $y = 3\sin\left(2x - \frac {3}{4}\pi \right)$;  
④ The range of the function $y = \cos(\sin x)$ is $[0, \cos 1]$.  
Among these, the correct propositions are \_\_\_\_\_\_\_\_\_\_\_\_．

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