Sn=(-1)^(1+n)×n!-(n-1)Sn-1,S1=1,S2=-3,S3=12,怎么求Sn?后面的Sn-1为一整体，下标号不会打

an=n(n+1) 求sn an=(-1)~n(n次方)*n,求sn Sn=n(n+2)(n+4)的分项等于1/6[n(n+2)(n+4)(n+5)-(n-1)n(n+2)(n+4)]吗? an是等差数列,求​lim (Sn+Sn+1)/(Sn+Sn-1)lim (Sn+Sn+1)/(Sn+Sn-1)=[n(n+1)/2+(n+1)(n+2)/2]/[n(n+1)/2+n(n-1)/2]=(2n²+4n+2)/2n²=1+2/n+1/n²我就想知道第一步怎么来的 求Sn=C(n,1)+2C(n,2)+...+nC(n,n)C(n,1)+2C(n,2)+...+nC(n,n) n是下标 Sn=1/2n∧2+1/2n 求sn/s(n+1) a1=1/2,Sn=n^2an-n(n-1),求Sn和an 已知Sn=2+5n+8n^2+…+(3n-1)n^n-1(n∈N*)求Sn sn=1*n+2(n-1)+3(n-2)+……+n*1 求和 求和：Sn=1*n+2*(n-1)+3*(n-2)+……+n*1 数列Cn=(n+2)/[n(n+1)]2^n的Sn 数列an=（(-1)^n + 4n）/2^n,求前n项和Sn 已知an=1/2n(n+1),求Sn an=(2n-1)*2^(n-1),求Sn an=(2n-1)2^n-1,求sn? sn=(n+1)/n an= 数列 an=(2n-1)3^n,Sn=? 知Sn=n²+2n+1求an