The natural numbers of 1, 2, 3 and 4. N are grouped according to the following rules: the first group: (1) the second group: (2, 3) the third group; (4, 5, 6) the fourth group: (7, 8, 9)

The natural numbers of 1, 2, 3 and 4. N are grouped according to the following rules: the first group: (1) the second group: (2, 3) the third group; (4, 5, 6) the fourth group: (7, 8, 9)

Observation can be found that the first group has one number, the second group has two numbers, and the third group has three numbers, so it is reasonable to speculate that the N group has n numbers. The last number of the first group is 1, the last number of the second group is 1 + 2, and the last number of the third group is 1 + 2 + 3, so it is reasonable to speculate that the last number of the N-1 group is 1 + 2 + 3 + +N-1 = n (n-1) / 2, the last number of group n is 1 + 2 + 3 + +N = n (n + 1) / 2, the first number of the nth group is n (n-1) / 2 + 1. The sum of the numbers of the nth group is n (n * 2 + 1) / 2
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