导读 An=nN+20+40+....20(n-1) =nN+10*(n-1)n =nN+10(n^2-n) n(N-10)+10n^2 SN=A1+A2+..+An =(N-10)(1+2+...+n)+10(1+2^2+3^3..+n^2) =5n(n+1)(2n+1)/3+n(n+1)(N-10)/2 =(n+1)(20n^2-20n+3N)/6。
An=nN+20+40+....20(n-1) =nN+10*(n-1)n =nN+10(n^2-n) n(N-10)+10n^2 SN=A1+A2+..+An =(N-10)(1+2+...+n)+10(1+2^2+3^3..+n^2) =5n(n+1)(2n+1)/3+n(n+1)(N-10)/2 =(n+1)(20n^2-20n+3N)/6。