1等差数列{an}的通项公式是an=2n+1,其前n项和为Sn,则数列{Sn/n}的前10项和为怎么来的2等比数列{an}中,a1=-1,前n项和为Sn,若S10/S5=31/32,则an等于().知道的写下大概过程就行了,

Question

1 等差数列{an}的通项公式是an=2n+1,其前n项和为Sn,则数列{Sn/n}的前10项和为
怎么来的
2等比数列{an}中,a1=-1,前n项和为Sn,若S10/S5=31/32,则an等于( )
.知道的写下大概过程就行了,

Answer 1

an = 2n + 1a1 = 3Sn = (a1 + an)*n/2 = n(n+2)Sn /n = n + 2bn = Sn /n = n + 2b1 = 3Tn = (b1 + bn)*n/2 = n(n+5)/2T10 = 10(10 + 5)/2 = 75--------------等比数列Sn = a1 * (q^n -1)/(q-1)S10/S5 = (q^10 -1)/(q...