
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 sequence ${a_n}$ satisfies $a_1=1$, $a_n+a_{n-1}=( \frac {1}{3})^{n}(n\geqslant 2)$, $S_n=a_1\cdot3+a_2\cdot3^{2}+…+a_n\cdot3^{n}$, find $4S_n-a_n\cdot3^{n+1}$ \_\_\_\_\_\_.

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