ANSWER:
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-++
STEP-BY-STEP EXPLANATION:
We can see that the first 3 steps:
[tex]\begin{gathered} s_1=1\cdot(1)=1 \\ s_2=2\cdot(1+2)=6 \\ s_1=3\cdot(1+2+3)=18 \end{gathered}[/tex]
Therefore, the formula could be the following
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