Finding the sum of the first n terms of sequence 3,7,11,15.2n-1

Finding the sum of the first n terms of sequence 3,7,11,15.2n-1

If the first term is 3, then the nth term = 2n + 1
Sum of the first n terms = (2n + 1 + 3) n / 2 = n (n + 2)

When a sequence has 2N-1 terms, its odd and even terms have several terms respectively

Because 2N-1 is an odd number and 2n-2 is an even number, the first 2n-2 items are just a pair, and the odd and even items are equal. The odd item has one more 2N-1 item

All odd numbers are arranged in a column from small to large, from which the number with 7 as divisor is deleted, and the remaining numbers form the sequence 1, 3, 5, 9, 11, 13, 15, 17, 19, 23
The number 2007 of this sequence is ()

Start the sequence from item 1 and group it into 6
2007 △ 6 = 334 + 3
The number of 2007 = 334 × 7 + 3 = 2341