
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

Find the sums:

a) $1 \cdot n+2(n-1)+3(n-2)+\ldots+n \cdot 1$.

b) $S_{n, k}=(1 \cdot 2 \cdot \ldots \cdot k) \cdot(n(n-1) \ldots(n-k+1))+(2 \cdot 3 \cdot \ldots \cdot(k+1)) \cdot((n-1)(n-2) \ldots(n-k))+\ldots+((n-k+1)(n-k+\ldots \cdot n) \cdot(k(k-1) \cdot \ldots \cdot 1)$.

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