a1+a2+a3=6
a2+a3+a4=q(a1+a2+a3)=3
所以q=1/2
a3+a4+a5+a6+a7+a8
=q^2(a1+a2+a3)+q^5(a1+a2+a3)
=6*[(1/2)^2+(1/2)^5]
=6*[(1/4)+(1/32)]
=27/16