Let {an} be an arbitrary equal ratio sequence, and the sum of the first n terms, the sum of the first 2n terms and the sum of the first 3N terms are x, y and Z respectively. Then () A. X+Z=2YB. Y（Y-X）=Z（Z-X）C. Y2=XZD. Y（Y-X）=X（Z-X）

Take the equal ratio sequence 1, 2, 4, let n = 1, then x = 1, y = 3, z = 7 are substituted into the checking calculation, only option D is satisfied, so select D

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1. Let {an} have 3N items in total, and the sum of the first 2n items is 100, and the sum of the last 2n items is 200. Then what is the sum of the middle n items?
2. It is known that the first n terms and Sn of sequence {an} are quadratic functions of N, and A1 = - 2, A2 = 2, A3 = 6 (1) Find Sn; (2) prove that sequence {an} is arithmetic sequence
3. Given that the first n terms and Sn of sequence {an} are quadratic functions of N, and its first three terms A1 = - 2, A2 = 2. A3 = 6, then A100 =?
4. It is known that the first n terms and Sn of sequence {an} are quadratic functions of N, and A1 = - 2, A2 = 2, A3 = 6. (1) find the expression of Sn; (2) find the general term an
5. Finite sequence 1,23,26,29 Why is the number of terms of 23n + 6 N + 3
6. In the arithmetic sequence 2,5,8 In, three numbers are inserted between two adjacent terms to form an arithmetic sequence, so that a new sequence can be obtained In the arithmetic sequence 2,5,8 In, three numbers are inserted between each adjacent two items to form an arithmetic sequence, so as to obtain a new sequence. Question: (1) what is the 12th item of the original sequence? (2) what is the 29th item of the new sequence?
7. If the sum of the first n terms is 25 and the sum of the first 2n terms is 100, then the sum of the first 3N terms is 100______ ．
8. In the known arithmetic sequence {an}, A1 = 2, A3 = 3, if three numbers are inserted between every two adjacent terms, it will form a new sequence with the number of the original sequence, Find 1) what is the 12th item of the original sequence? 2) whether the 29th item of the new sequence is the item of the original sequence? If so, what is it?
9. It is known that {an} is an arithmetic sequence, A1 = 2, A2 = 3. If three numbers are inserted between every two adjacent terms to form a new arithmetic sequence with the number of the original sequence, we can find: (1) what is the 12th term of the original sequence? (2) What is the 29th item of the new sequence?
10. If the sum of their first n terms is Sn / TN = 3N + 1 / 2N-1, then the ratio of the fifth term of these two numbers is?
11. Let {an} be an arbitrary equal ratio sequence, the sum of the first n terms, the sum of the first 2n terms and the sum of the first 3N terms be x, y and Z respectively, then () A. x+z=2yB. y（y-x）=z（z-x）C. y2=xzD. y（y-x）=x（z-x）
12. Let {an} be an arbitrary equal ratio sequence, the sum of the first n terms, the sum of the first 2n terms and the sum of the first 3N terms be x, y and Z respectively, then () A. x+z=2yB. y（y-x）=z（z-x）C. y2=xzD. y（y-x）=x（z-x）
13. If the general term formula of sequence {an} is a quadratic function of N and A1 = 2, A2 = 6, A3 = 12, find the general term formula of {an} N is under a
14. 3*（1+2+3+4+.n）-n =3*（1+n）*n/2-n =(3n^2+n)/2 How to simplify to the last step
15. For any natural number n, the ratio of the sum of the first n terms to the sum of the first 2n terms is a fixed value, then the general term of an
16. If a is a natural number that is not zero, what is 8 / 1 △ a 1 / 8 △ a = how much? 1 / a △ 8 = how much?
17. N is a natural number (n is not equal to 0). What are the two adjacent natural numbers?
18. Fill in the blanks with the verb be. This pair of shoes -- for Nick Then the shoes - whirt
19. There is a round flower bed in the park, with a circumference of 50.24 meters. There is a 1 meter wide path around it. How many square meters is the area of the path?
20. Plant the seeds in the soil