等差数列的前十项之和为100，前100项和为10，这前110项之和为？ 等差数列，a1+a3+a9=9,a6=9,则前6项之和为

(1)an=a1+(n-1)d
Sn=na1+n(n-1)d/2
所以:S10=10a1+10*(10-1)d/2=100
a1+9d/2=10---------(1)
S100=100a1+100(100-1)d/2=10
10a1+990d/2=1--------(2)
解(1),(2)得:
a1=1099/100
d=-11/50

S110=110a1+110(110-1)d/2
=-110

(2)an=a1+(n-1)d
a3=a1+2d
a9=a1+8d
a1+a3+a9=3a1+11d
3a1+11d=9--------(1)
a6=a1+5d
a1+5d=9----------(2)
解(1),(2)得:
d=9/2
a1=-27/2

s6=6a1+6(6-1)d/2=6(-27/2)+30(9/2)/2=-27/2