3*（1+2+3+4+.n）-n =3*（1+n）*n/2-n =(3n^2+n)/2 How to simplify to the last step

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• 1. 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
• 2. 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）
• 3. 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）
• 4. 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）
• 5. 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?
• 6. 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
• 7. 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 =?
• 8. 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
• 9. Finite sequence 1,23,26,29 Why is the number of terms of 23n + 6 N + 3
• 10. 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?
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• 20. English translation Hands on brow bones - shame Tight lips - embarrassment Scratching your head - at a loss Repeated rubbing of hands - anxiety Playing with ties or accessories - restless Unfocused, left and right - scared Arms in arms - self protection, uneasiness Hands in belt and shoulders up - tense