（2-3×5-1）+（4-3×5-2）+… （2n-3×5-n）=______ ．

（2-3×5-1）+（4-3×5-2）+… （2n-3×5-n）=______ ．

The original formula = (2 + 4 +...) +2n）-3×（5-1+5-2+… +5-N) = n (2 + 2n) 2-3 × 5 − 1 (1 − 5 − n) 1 − 5 − 1 = n (n + 1) - 3 × 1 − 5 − N4 = n (n + 1) - 34 [1 - (15) n]

Sum: 1 + (2n + 3) + (4N + 5) +. + 2n ^ 2-1=
1+(2n+3)+(4n+5)+.+2n^2-1=
That's what I did
sn= 1+7+17+.+(2n^2-1)
sn={2+8+18+.+2n^2}-n
sn-n=2{1+2^2+3^2+.+n^2}
sn=n(n+1)(2n+1)/3+n
The result is wrong. Which step is wrong

sn-n=2{1+2^2+3^2+.+n^2}
It should be Sn + n = 2 {1 + 2 ^ 2 + 3 ^ 2 +. + n ^ 2}
The result is n (n + 1) (2n + 1) / 3-N

Given the first four terms of the sequence: 1,2 / 3,1 / 2,2 / 5, write a general term formula:

an=2/(n+1)