The sum of the first n terms of sequence 11 + 2, 11 + 2 + 3, 11 + 2 + 3 + 4, a is______ ．

The sum of the first n terms of sequence 11 + 2, 11 + 2 + 3, 11 + 2 + 3 + 4, a is______ ．

Because an = & nbsp; 11 + 2 + 3 + +(n+1)＝2(n+2)(n+1)=2(1n+1−1n+2)Sn=2(12−13+13−14+… +11 + n − 1n + 2) = 2 (12 − 1n + 2) = NN + 2, so the answer is: NN + 2

Group summation formula, examples and scope of application

S = x + 2x ^ 2 + 3x ^ 3. + NX ^ n there are two summation methods. Because the coefficient is the arithmetic sequence and the unknown is the proportional sequence, the dislocation subtraction can be used. But there is a better method, which is the split term method. The key of the split term method is to disassemble the general term

Examples and exercises are the best,

On the first floor, "if the sequence is an equal ratio sequence, you can use the inverse system (order) summation method" is wrong. It should be that the arithmetic sequence can use the inverse order summation method N find the sum of the first n terms 1, 2, 3, 4 nn n-1 n-2 n-3…… Let s be the sum of the first n terms, and 2S = (n + 1) + [(n