3, 9, 18, 30?

First of all, we know the meaning of this sequence: it is the sum of the first n terms of the sequence an = 3N. So it is very simple. The general formula an = 3N, Sn = [(3 + 3n) * n] / 2

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1. -3,6, - 9,12, - 15,18, - 21. Expressed by the algebraic formula containing n
2. 3, 9, 18, 30, 45, 63 are expressed by the algebraic formula containing X
3. If the values of algebraic formula 12-3 (9-y) and algebraic formula 5 (y-4) are equal, then y = 0___ ．
4. It is expressed by the algebraic formula containing N: 1,2,3,4,5 ；1,3,5,7,9
5. Carefully observe the arrangement rule of the sequence, and choose the correct one to make it conform to the arrangement rule of the original sequence: 4,38,18,10,4, () A：2 B：1 C：3
6. Please find the sequence 18,34,55,81 , 1 / 2 [(n-3) (5N + 12)] + 18 Please find the sequence 18,34,55,81 , 1 / 2 [(n-3) (5N + 12)] + 18. The rule of this sequence is that the difference between them is an arithmetic sequence,
7. Sequence: 4,5,6,8,9,10,12,13,14,... What are the rules?
8. Find the law, sequence 3,4,9,13
9. Finding the law of sequence 1,3,6,10,15.……………… In the same plane, when two straight lines intersect, there is a point, when three lines intersect, there are three corners, when four lines intersect, there are six How many intersections can n (n is greater than 1) lines have at most
10. Find the law of sequence: 5,5,14,38,87, (), what's the next one?
11. 1,-1,-2,3,-5,-8,13,（）,（）
12. Please fill in 3, 8, 10, 18, 17, 28, (), (), (), ()
13. Fill in the numbers according to the rules (18,17), (14,10), (10,1), (?, 5), and one more question (1,3) (5,9) (7,13) (9,?)
14. First 23 * (- 5) - (- 3) / 123 score 8 second - 7 * (- 3) * (- 0.5) + (- 12) * (- 2.6)
15. 1×12÷323÷1×4×2×2×0= 2×3×4×5×6×7×8×9×123÷6=
16. 11 / 12 + 3 / 7 1 + 3 / 12 1 + (5 / 7 6-0.123)
17. (1+12+123+1234+.+123.56789)÷4
18. 1+12+123+1234=? 1+12+123+1234=?
19. What is the decimal expression of 123-7 (123 out of seven)?
20. ｛100+123｝/12