Question

求和：Sn=1+2x3+3x5+……+nx(2n-1）

Answer 1

an=n(2n-1)=2n²-n

所以Sn=2(1²+2²+……+n²)-(1+2+……+n)

=2n(n+1)(2n+1)/6-n(n+1)/2

=n(n+1)(4n-1)/6