an =a1+(n-1)d
Sn = [2a1+(n-1)d]n/2
bn = Sn/n
=[2a1+(n-1)d]/2
=a1 +(n-1)(d/2)
{bn}是等差数列, b1=a1, 公差=d/2
S7=7
a1+3d =1 (1)
S15=75
a1+7d= 5 (2)
(2)-(1)
d=1
from (1) , a1=-2
an = -2+(n-1)= n-3
Sn = (n-5)n/2
bn = Sn/n = (n-5)/2
Tn =b1+b2+...+bn
= (n-11)n/4
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