4a7=a1+3a4
4a1*q^6=a1+3a1*q^3
4q^6=1+3q^3
4q^6-3q^3-1=0
(q^3-1)(4q^3+1)=0
q^3=-1/4
Sn=[a1(1-q^n)]/(1-q)
S6/12S3=(1-q^6)/12(1-q^3)=(1-1/16)/12[1-(-1/4)]=1/16
(S12-S6)/S6=[(1-q^12)-(1-q^6)]/(1-q^6)=[1-1/256-1+1/16]/(1-1/16)=(15/256)/(15/16)=1/16
12S3,S6,S12-S6成等比