题是求和:Sn=1×3+2×3∧2+3×3∧3+…+n×3∧n
求和:Sn=1*n+2*(n-1)+3*(n-2)+……+n*1
求和:Sn=1*2*3+2*3*4+……+n(n+1)(n+2)
求和sn=1×2×3+2×3×4+……+n(n+1)(n+2)
数列求和:sn=1+1/2+1/3+…+1/n,求sn
求和:Sn=1*2+1*2^2+3*2^3+……+n*2^n.
求和Sn=(a-1)+(a^2-2)+(a^3-3)+…+(a^n-n)?
求和Sn=1/a+2/a^2+3/a^3+…+n/a^n
数列求和:Sn=1/1*2*3+1/2*3*4+.+1/n*(n+1)*(n+2) 求Sn
求和:Sn=1-3x+5x^2-7x^3+.+(2n+1)(-x)^n(n属于N*)
\求和Sn=1+2x+3x^2+```+(n-1)x^(n-2)+n*x^(n-1)
数列求和 用分组求和及并项法求和 Sn=1^2-2^2+3^2-4^2+…+(-1)^(n-1)·n^2
Sn=1/3+2/3²+3/3³.n/3的n次方 求和!