Explore the law: the first is 1, the second is 4, the third is 13, the fourth is 40, the fifth is 121, find the nth

The rule is: the sum of the first n terms of the equal ratio sequence whose first term is 1 and equal ratio is 3
Therefore, item n is:
1*（1-3^n）/(1-3)=(3^n-1)/2

1, 4, 13, 40, 81, 121 what's the law?

4-1 = 3 13-4 = 9 40-13 = 27 81-27 = 54 let's see what the rules are
Perhaps the biggest rule is that he never has a rule

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